Include pkg_resources test data in sdist

Fix broken link to distutils docs for package_data

Use CPython 3.8.0 mechanism to find msvc 14+

Add news fragment

The broken link used to point to Python 2 docs (which has been moved).
Since Python 2 has reached its end-of-life, link to Python 3 docs instead.

Copy docs requirements to a separate file due to RTD constraints.

Copy docs requirements to a separate file due to RTD constraints. Fixes #2001. Ref readthedocs/readthedocs.org#6662

Pin virtualenv to <20 as workaround for #1998.

Avoid installing setuptools 45 on Python 2.

Because Requires-Python is disabled for Python 2 to allow this declared-incompatible version to install on Python 2, the build-requirements must exclude installing setuptools 45 during the build-system setup.

Because Python 2 tests are only needed for internal validation, and because the latest pip is already installed, the build-system can rely on `backend-path = ["."]`.

Fixes #1996.

Deprecated Eggsecutable Scripts

This is the error otherwise:

_____________ TestFindDistributions.test_standalone_egg_directory ______________

self = <pkg_resources.tests.test_find_distributions.TestFindDistributions object at 0x7f17518a86d0>
target_dir = local('/tmp/pytest-of-mockbuild/pytest-0/test_standalone_egg_directory0/target')

    def test_standalone_egg_directory(self, target_dir):
        (TESTS_DATA_DIR / 'my-test-package_unpacked-egg').copy(target_dir)
        dists = pkg_resources.find_distributions(str(target_dir))
>       assert [dist.project_name for dist in dists] == ['my-test-package']
E       AssertionError: assert [] == ['my-test-package']
E         Right contains one more item: 'my-test-package'
E         Use -v to get the full diff

pkg_resources/tests/test_find_distributions.py:25: AssertionError
____________________ TestFindDistributions.test_zipped_egg _____________________

self = <module 'py.error'>, func = <built-in function listdir>
args = ('/builddir/build/BUILD/setuptools-45.2.0/pkg_resources/tests/data/my-test-package_zipped-egg',)
kwargs = {}, __tracebackhide__ = False, cls = <class 'py.error.ENOENT'>
value = FileNotFoundError(2, 'No such file or directory')
tb = <traceback object at 0x7f1751a6a7c0>, errno = 2

    def checked_call(self, func, *args, **kwargs):
        """ call a function and raise an errno-exception if applicable. """
        __tracebackhide__ = True
        try:
>           return func(*args, **kwargs)
E           FileNotFoundError: [Errno 2] No such file or directory: '/builddir/build/BUILD/setuptools-45.2.0/pkg_resources/tests/data/my-test-package_zipped-egg'

/usr/lib/python3.8/site-packages/py/_error.py:66: FileNotFoundError

During handling of the above exception, another exception occurred:

self = <pkg_resources.tests.test_find_distributions.TestFindDistributions object at 0x7f1751c8fe80>
target_dir = local('/tmp/pytest-of-mockbuild/pytest-0/test_zipped_egg0/target')

    def test_zipped_egg(self, target_dir):
>       (TESTS_DATA_DIR / 'my-test-package_zipped-egg').copy(target_dir)

pkg_resources/tests/test_find_distributions.py:30:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/usr/lib/python3.8/site-packages/py/_path/local.py:437: in copy
    for x in self.visit(rec=rec):
/usr/lib/python3.8/site-packages/py/_path/common.py:377: in visit
    for x in Visitor(fil, rec, ignore, bf, sort).gen(self):
/usr/lib/python3.8/site-packages/py/_path/common.py:414: in gen
    entries = path.listdir()
/usr/lib/python3.8/site-packages/py/_path/local.py:392: in listdir
    names = py.error.checked_call(os.listdir, self.strpath)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <module 'py.error'>, func = <built-in function listdir>
args = ('/builddir/build/BUILD/setuptools-45.2.0/pkg_resources/tests/data/my-test-package_zipped-egg',)
kwargs = {}, __tracebackhide__ = False, cls = <class 'py.error.ENOENT'>
value = FileNotFoundError(2, 'No such file or directory')
tb = <traceback object at 0x7f1751a6a7c0>, errno = 2

    def checked_call(self, func, *args, **kwargs):
        """ call a function and raise an errno-exception if applicable. """
        __tracebackhide__ = True
        try:
            return func(*args, **kwargs)
        except self.Error:
            raise
        except (OSError, EnvironmentError):
            cls, value, tb = sys.exc_info()
            if not hasattr(value, 'errno'):
                raise
            __tracebackhide__ = False
            errno = value.errno
            try:
                if not isinstance(value, WindowsError):
                    raise NameError
            except NameError:
                # we are not on Windows, or we got a proper OSError
                cls = self._geterrnoclass(errno)
            else:
                try:
                    cls = self._geterrnoclass(_winerrnomap[errno])
                except KeyError:
                    raise value
>           raise cls("%s%r" % (func.__name__, args))
E           py.error.ENOENT: [No such file or directory]: listdir('/builddir/build/BUILD/setuptools-45.2.0/pkg_resources/tests/data/my-test-package_zipped-egg',)

/usr/lib/python3.8/site-packages/py/_error.py:86: ENOENT

Bootstrap the environment in tox, allowing simple 'tox' to run tests and simplifying all of the pipelines.

Make easy_install command less strict (fixes #1405)

Fix install_scripts() if bdist_wininst is missing

Move test dependencies to package metadata.

Closes #1985

Move docs dependencies into package metadata and use same technique as jaraco/skeleton to build docs in tox and rtd.

Improve tests setup in GH Actions CI/CD workflow

Opt into PEP 517 for pip installs to avoid implicit dependency on Setuptools to install test dependencies.

Move test dependencies into the package, removing 'tests' directory which masks the error reported in #1896.

Rewrite selective PIP_IGNORE_REQUIRES_PYTHON to allow test dependencies in metadata to be installed without ignoring REQUIRES_PYTHON