Support requirement parsing in setuptools.build_meta

Since pkg_resources is imported elsewhere anyway, we don't get much
value out of porting the requirement parser locally.

This fixes GH #1682 by porting the pkg_resources requirement parsing
logic into setuptools.build_meta, so that all valid requirement
specifiers passed to setup_requires will be added to the
get_requires_for_build_* function outputs.

Fixes GH #1682

Per GH #1682, with setuptools.build_meta we are not properly handling
the situation where setup_requires is actually a newline-delimited
string rather than a list, which is supported by setup.py interface.

This adds several failing (and some passing) tests for how
setup_requires is handled by setuptools.build_meta.

Found by lgtm

These sections of the documentation were never written, and refer to deprecated functionality anyway, so they can be removed.

tests: improve `test_pip_upgrade_from_source`

Parametrize the test to check different versions of pip
(including master) are correctly supported.

Move version to declarative config

Improve badges in README

Move bumpversion config to .bumpversion.cfg as it seems not to be possible to handle setup.cfg. Ref c4urself/bump2version#62.

* Consistent Styling
* Add Logos of CI services
* Shorten CI badge labels
* Reorder for better wrapping

Add support for setup.cfg-only projects

Many projects can get away with an empty `setup.py` and use *only* the
declarative `setup.cfg`. With the new PEP 517 backend, we can supply a
default empty `setup.py` if one is not provided.

Update changelog for version 40.8.0

Add build_meta_legacy backend

Rather than exposing a top-level module for the legacy backend, we will
move the legacy backend into the `setuptools.build_meta` module and
specify it using the module:object syntax.

Using the absolute path to the directory of the setup script better
mimics the semantics of a direct invocation of python setup.py.

Per Nick Coghlan's suggestion on PR #1652, a try/finally block ensures
that the path is restored even in the event of an error.

This is part of the solution to GH #1642, it is a
backwards-compatibility backend that can be used as a default PEP 517
backend for projects that use setuptools but haven't opted in to
build_meta.

This runs all build_meta tests, plus a test that it is possible to
import from the directory containing `setup.py` when using the
build_meta_legacy backend.

Failing test adapted from PR #1643
Co-authored-by: Tzu-ping Chung <uranusjr@gmail.com>

In order to support both the `build_meta` and `build_meta_legacy`
backends, the core functionality is wrapped in a class with methods to
be overridden in build_meta_legacy. The class is an implementation
detail and should remain private.

Bump version: 40.7.2 → 40.7.3