fix(pytest_plugin): Pytester import, use public APIs from pytest

[tony]

Fixes (#441)

Avoid using internal pytest APIs for typing:

• _pytest.pytester.Pytester -> pytest.Pytester
• _pytest.monkeypatch.MonkeyPatcher -> pytest.MonkeyPatch
• _pytest.doctest.DoctestItem -> ??? (see Public API for DoctestItem pytest-dev/pytest#10312)
• add conftest.py to sdist

[codecov]

Codecov Report

Merging #442 (f4d5bfc) into master (9828540) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #442      +/-   ##
==========================================
- Coverage   86.34%   86.27%   -0.07%     
==========================================
  Files          20       20              
  Lines        1823     1821       -2     
  Branches      277      277              
==========================================
- Hits         1574     1571       -3     
- Misses        170      171       +1     
  Partials       79       79              

Impacted Files	Coverage Δ
tests/test_pytest_plugin.py	100.00% <ø> (ø)
tests/test_common.py	97.43% <100.00%> (-0.03%)	⬇️
src/libtmux/session.py	74.86% <0.00%> (-0.56%)	⬇️

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