mypy errors every other run

[gitpushdashf]

Started getting this when upgrading from 0.812 to 0.902.

This is using Poetry and namespace_packages set to True. Otherwise, a pretty minimal mypy.ini.

python % poetry run mypy --strict
~/Library/Caches/pypoetry/virtualenvs/hos-dev-C-FUiXg3-py3.8/lib/python3.8/site-packages/google/fhir/_primitive_time_utils.py:34: error: Library stubs not installed for "backports" (or incompatible with Python 3.8)
~/Library/Caches/pypoetry/virtualenvs/hos-dev-C-FUiXg3-py3.8/lib/python3.8/site-packages/google/fhir/_primitive_time_utils.py:34: note: Hint: "python3 -m pip install types-backports"
~/Library/Caches/pypoetry/virtualenvs/hos-dev-C-FUiXg3-py3.8/lib/python3.8/site-packages/google/fhir/_primitive_time_utils.py:34: note: (or run "mypy --install-types" to install all missing stub packages)
~/Library/Caches/pypoetry/virtualenvs/hos-dev-C-FUiXg3-py3.8/lib/python3.8/site-packages/google/fhir/_primitive_time_utils.py:34: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 88 source files)
python % poetry run mypy --strict
Success: no issues found in 88 source files
python % poetry run mypy --strict
~/Library/Caches/pypoetry/virtualenvs/hos-dev-C-FUiXg3-py3.8/lib/python3.8/site-packages/google/fhir/_primitive_time_utils.py:34: error: Library stubs not installed for "backports" (or incompatible with Python 3.8)
~/Library/Caches/pypoetry/virtualenvs/hos-dev-C-FUiXg3-py3.8/lib/python3.8/site-packages/google/fhir/_primitive_time_utils.py:34: note: Hint: "python3 -m pip install types-backports"
~/Library/Caches/pypoetry/virtualenvs/hos-dev-C-FUiXg3-py3.8/lib/python3.8/site-packages/google/fhir/_primitive_time_utils.py:34: note: (or run "mypy --install-types" to install all missing stub packages)
~/Library/Caches/pypoetry/virtualenvs/hos-dev-C-FUiXg3-py3.8/lib/python3.8/site-packages/google/fhir/_primitive_time_utils.py:34: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 88 source files)
python % poetry run mypy --strict
Success: no issues found in 88 source files

You can see how every other run it picks up that error. Very weird, not sure what to make of it.

[hauntsaninja]

Probably the same as #9852

[gitpushdashf]

Thank you, that is helpful.

[hauntsaninja]

Fixed by #10937