Implement MypyItem.collect for pytest < 6.0

[dmtucker]

Resolve #115

• This partially works around Doctest collection only returns single test for __init__.py pytest-dev/pytest#8016 for 3.10 <= pytest < 6.0.
  • It avoids AttributeError: 'MypyStatusItem' object has no attribute 'collect' - running pytest 5.4 with looponfail  #115, but we cannot prevent taking a code path that only collects 1 Item.
• The workaround does not help with 6.0 <= pytest < 6.2 (because an assertion fails first).
• The issue is truly fixed in pytest 6.2 (by main: fix only one doctest collected on pytest --doctest-modules __init__.py pytest-dev/pytest#8022).