Fix some usage of deprecated `imp` module

This reverts commit 42bd00ad.

travis: update PyPy jobs to use more recent versions

tests: fix `test_pip_upgrade_from_source` on Python 3.4

Do not test pip's master on 3.4, as support for it has been dropped.

Perform a more thorough check of package_data structure

Ensure the dictionary values are lists/tuples of strings.

Fix #1459.

value=None raises TypeError

DistutilsSetupError: 2 must be a list of strings (got None)

Include the file path in get_metadata() UnicodeDecodeErrors.

Fix CI

Use license classifiers rather than the license field.

stop using deprecated HTMLParser.unescape

HTMLParser.unescape is accessed even when unused - this will cause an
exception when `HTMLParser.unescape` is removed in Python 3.9.

The license field has a 'free' format, while the classifiers are unique
identifiers, similar to SPDX identifiers. In the documentation, we
should therefore showcase the use of classifiers.

tests: unpin pytest

The new releases for pytest-fixture-config and pytest-shutil are
compatible with pytest>=4.0.

Force metadata-version = 1.2 when project urls are present.

Closes: #1756

Migrate constants from setup.py to setup.cfg

This also makes wheel an unconditional setup_requires dependency.

Closes: #1697

Fix CIs.

Fix the `list_dependencies_command` so arguably the most important
packages are not hidden (pip, setuptools, and wheel).

Update virtualenv so the tox environment use a more recent version
of pip (and probably setuptools/wheel too).

* ignore distutils' warning (`Unknown distribution option: 'python_requires'`)
* fix test on Windows

build_meta: fix 2 issues with `build_wheel` / `build_sdist`

Fix the following cases:

* `build_sdist` is called with another sdist already present
  in the destination directory

* `build_wheel` is called with the destination directory not
  already created