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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.

tweak #1644 workaround to fix `test_pip_upgrade_from_source`

Bugfix/1663 restore port

Ensure port is included in package-index

Ensure a specified port in package_index isn't lost in the parse/unparse of the URL when auth is present. Fixes #1663.