Add support for pytest-xdist

[dmtucker]

Resolve #72

#41 changed the plugin significantly by calling mypy once per session on all MypyItems instead of once per MypyItem. Unfortunately, that broke compatibility with pytest-xdist in 2 ways:

• It used pytest_runtestloop, which is re-implemented by pytest-xdist workers.
• It didn't expect terminal could be None.

To remedy the latter, this moves mypy unmatched output and stderr reporting to pytest_terminal_summary. For the former, this combines the following recipes:

• "Making session-scoped fixtures execute only once" from pytest-xdist
  • The first MypyItem to execute runs mypy and writes the parsed results to a temporary file.
• "Shared Directory Between Master and Workers With pytest-xdist" from pytest-tricks
  • The path to the file is shared between master (for pytest_terminal_summary) and workers (to read the results).
• "Optionally using hooks from 3rd party plugins" from pytest
  • The path is only shared if pytest-xdist is enabled.