When running in a complex environment with lots of installed
packages, PYTHONPATH gets way too long. Instead, just make sure
that paths_on_pythonpath doesn't contain duplicates

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

Revert to using a copy of splituser from Python 3.8.

Revert to using a copy of splituser from Python 3.8. Using urllib.parse.urlparse is clumsy and causes problems as reported in #1663 and #1668. Alternative to #1499 and fixes #1668.

Drop build-backend from pyproject.toml

Because we do not include setuptools in our build requirements and there
is no mechanism for a PEP 517 backend to bootstrap itself, setuptools
cannot use setuptools.build_meta as its backend, and cannot use PEP 517
to build itself.

Additionally, if you specify build-backend, it is impossible to disable
PEP 517 builds with pip's --no-use-pep517 flag.