tests: temporary pin pip to fix the Python 2.7 job

Pin pytest to <4 for now. Ref #1638.

In build_meta (PEP 517), always built gztar

Refactor build_meta

Always specify formats=gztar, overriding the project's legacy expectation that a zip sdist should be generated. Fixes #1623.

Tweak "making a pull request" documentation

reStructuredText is how the name is commonly stylized (see Wikipedia),
also fixed a typo in the "deprecation" description.

Spelling fixes

Disallow files for license inputs

Both the old and new approaches are deeply unsatisfying to me, but
without reworking how these test commands are run, I think this is
about as close as we can get to enforcing that this specific call
raises ValueError.

The ability to handle files was originally added and documented based on
a misunderstanding of what the `license` field should include. The field
should be the name of the license, not the full text.

It is likely that anyone actually using this was outputing malformed
PKG-INFO files, because most license files contain newlines.

See GH issue #1551

Use HTTPS in link to RFC #6125 (in a private method's docstring)

Add documents on auto-merge

Remove restriction for jaraco - only members with access can create labels, so it's secure enough to restrict based on the label.

Only allow auto-merge PRs submitted by authorized users

Add declaration indicating the PEP 517 build backend needed for building setuptools.