Fix Requires metadata handling.

[jsirois]

We now warn when encountering legacy Requires metadata but otherwise
ignore it in line with the PyPA tools.

Fixes #1201

[jsirois]

Hrm, a fair bit of 502's. python.org looks nominally OK - but I'll give this a break for a while before re-running.

Reviewers: this is good to go and passes the relevant bits on my machine:

$ tox -epy{py2,py3,27,3{5,6,7,8,9}} -p -- -vvsk test_requires_dists_none
✔ OK py27 in 22.295 seconds
✔ OK py39 in 22.374 seconds
✔ OK py37 in 24.941 seconds
✔ OK py38 in 25.166 seconds
✔ OK py35 in 34.992 seconds
✔ OK py36 in 36.366 seconds
✔ OK pypy2 in 41.017 seconds
✔ OK pypy3 in 48.922 seconds
__________________________________________________________________________________________________________________________________________ summary ___________________________________________________________________________________________________________________________________________
  pypy2: commands succeeded
  pypy3: commands succeeded
  py27: commands succeeded
  py35: commands succeeded
  py36: commands succeeded
  py37: commands succeeded
  py38: commands succeeded
  py39: commands succeeded
  congratulations :)

$ tox -epy{py2,py3,27,39}-integration -p -- -vvsk test_requires_metadata_issues_1201
✔ OK py27-integration in 8.713 seconds
✔ OK py39-integration in 11.513 seconds
✔ OK pypy2-integration in 29.028 seconds
✔ OK pypy3-integration in 38.217 seconds
__________________________________________________________________________________________________________________________________________ summary ___________________________________________________________________________________________________________________________________________
  pypy2-integration: commands succeeded
  pypy3-integration: commands succeeded
  py27-integration: commands succeeded
  py39-integration: commands succeeded
  congratulations :)

[jsirois]

Good to go - PyPI has de-flaked.