A fair amount of the packages in third_party/python/ have been copied in there manually, causing two downsides:

• They aren't asserted to be compatible with each other
• It's not obvious what some of the versions of the packages are

We should port them to use the centralized system.

You can probably also remove the warning while you're here.

There's no usages of the pep487 module.

Depends on D123120

Note that, as part of adding this packages to the automated vendoring
system, some dependencies were automatically added - most notably,
dependencies of taskcluster that become visible with Python 3.6+.

Adds **/.git to the exclusions because:

• .git is part of our .hgignore, but
• .git is part of the aiohttp tar.gz file.

Since the file isn't needed for pip install-ing aiohttp,
and since we want ./mach vendor python to be a no-op when there's
no requirement changes, we exclude it.

Depends on D123121

Depends on D123122

Rather than deleting the expected target directory of each package
that's being vendored, clear the whole third_party/python directory
and re-populate it from scratch.

As part of this, there's an "exclusion" list for packages that can't
be vendored from PyPI.

This has some benefits:

• It'll be harder to forget scraps of files and directories and leave
  them in third_party/python.
• The exclusion list makes it more clear which packages are managed
  manually, and the friction it adds to the workflow will guide
  developers to use "requirements.in" instead.

The test_up_to_date_vendor test will verify that the vendor directory
is always clean.

Depends on D123123

Backed out for causing invalid handle xpcshell failures.

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=U48eqelWQge_ZBWzSGPX-A.0&resultStatus=testfailed%2Cbusted%2Cexception%2Cusercancel&revision=90b9ff20b311609163c786cdee830ff9a132d575

Failure log: https://treeherder.mozilla.org/logviewer?job_id=350477627&repo=autoland

Backout link: https://hg.mozilla.org/integration/autoland/rev/6b58b83b626e01f7b2af85156c293c9bccd82816

https://hg.mozilla.org/mozilla-central/rev/746509f3a640
https://hg.mozilla.org/mozilla-central/rev/06115f01c86f
https://hg.mozilla.org/mozilla-central/rev/dd841f22c83e