pytest: Better API for assertion failure reprs

PyCharm injects a pytest plugin to enable the IDE to intercept comparison assertion failures and provide an integrated diff UI for them. Currently it does a bunch of messy things that fail in complex cases:

• implements pytest_assertrepr_compare to encode the assertion failure in a more resilient way
• overrides config.option.verbose = 2 to avoid truncation
• picks up the results in pytest_runtest_logreport
• hackily undoes _pytest.assertion.util.format_explanation

It’ll be nice to have a way for plugins to stash assertion failures in a more structured way so they could be “picked up” in pytest_runtest_logreport cleanly and then serialized in whatever way the IDE prefers to get them.

Currently:

• pytest_assertrepr_compare gets config but not item so there’s no way to stash structured results on the item
• pytest_assertrepr_compare return value is a Optional[List[str]] (no complex data structure)

Open to other ideas of course.