Add warning for assertions on tuples #1562

[kvas-it]

This PR adds a warning for assertions on tuples. The warning is issued by the assertion rewriter, as suggested by @RonnyPfannschmidt, so it will only be shown if the asserts are being rewritten (which is the default in most cases).

Checklist:

• Target: for bug or doc fixes, target master; for new features, target features
• Make sure to include one or more tests for your change
• Add yourself to AUTHORS
• Add a new entry to the CHANGELOG (choose any open position to avoid merge conflicts with other PRs)

[coveralls]

Coverage increased (+0.007%) to 92.435% when pulling 0db4ae1 on kvas-it:issue-1562 into 68bed00 on pytest-dev:features.

[The-Compiler]

Could you add a test to show something like this does not show the warning?

tpl = ('foo', 'bar')
assert tpl

[coveralls]

Coverage increased (+0.007%) to 92.435% when pulling 6d4cee2 on kvas-it:issue-1562 into 68bed00 on pytest-dev:features.

[The-Compiler]

LGTM, but I'd welcome a second pair of eyes by someone who's more familiar with that code.

[nicoddemus]

Thanks @kvas-it! 😄