Mypy test failure

[matthewgilbert]

For python 3.9 it appears a new package is necessary for mypy typing

Run pytest
============================= test session starts ==============================
platform linux -- Python 3.9.6, pytest-6.2.4, py-1.10.0, pluggy-0.13.1
rootdir: /home/runner/work/blp/blp, configfile: pyproject.toml, testpaths: src
plugins: isort-2.0.0, black-0.3.12, mock-3.6.1, pycodestyle-2.2.0, mypy-0.8.0, flakes-4.0.3, pydocstyle-2.2.0
collected 114 items / 58 deselected / 56 selected

src/blp/__init__.py ..F...                                               [ 10%]
src/blp/blp.py .F....ss....                                              [ 32%]
src/blp/test/__init__.py .....                                           [ 41%]
src/blp/test/test_blp.py .F...............................               [100%]

=================================== FAILURES ===================================
_________________________________ test session _________________________________
mypy exited with status 1.
________________________________ src/blp/blp.py ________________________________
11: error: Library stubs not installed for "pytz" (or incompatible with Python 3.9)
11: note: Hint: "python3 -m pip install types-pytz"
11: note: (or run "mypy --install-types" to install all missing stub packages)
11: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
___________________________ src/blp/test/test_blp.py ___________________________
10: error: Library stubs not installed for "pytz" (or incompatible with Python 3.9)
===================================== mypy =====================================
Found 2 errors in 2 files (checked 4 source files)
=========================== short test summary info ============================
FAILED src/blp/__init__.py::mypy-status
FAILED src/blp/blp.py::mypy
FAILED src/blp/test/test_blp.py::mypy
=========== 3 failed, 51 passed, 2 skipped, 58 deselected in 44.65s ============
Error: Process completed with exit code 1.

https://github.com/matthewgilbert/blp/runs/3311910147