…#2831)

This just adds the code back at the original state before the following
PRs have been made to remove them: bazel-contrib#2629, bazel-contrib#2781. This has not been
hooked up yet in `evaluate_markers` and `whl_library` yet and I'll need
extra PRs to do that.

No CHANGELOG entries for now, will be done once the integration is back.

Work towards bazel-contrib#2830

)

Handling of `python_full_version` correctly has been fixed in the
Starlark
implementation in bazel-contrib#2793 and in this PR I am backporting the changes to
handle
the full python version target platform strings so that we can have the
same
behaviour for now.

At the same time I have simplified and got rid of the specialization
handling
in the Python algorithm just like I did in the starlark, which
simplifies the
tests and makes the algorithm more correct.

Summary:
* Handle `cp3x.y_os_arch` strings in the `platform.py`
* Produce correct strings when the `micro_version` is unset. Note, that
we use version `0` in evaluating but we use the default version in the
config setting. This is to keep compatibility with the current behaviour
when the target platform is not fully specified (which would be the case
for WORKSPACE users).
* Adjust the tests and the code to be more similar to the starlark impl.

Work towards bazel-contrib#2830

…contrib#2834)

Summary:
- Revert to using Python for marker evaluation during parsing of
  requirements (partial revert of bazel-contrib#2692).
- Use Python to parse whl METADATA.
- Bugfix the new simpler algorithm and add a new unit test.

Fixes bazel-contrib#2830