Enable the use of Mypy 0.900+

Mypy would complain about the missing `types-mock` package, which it now
needs to perform accurate type checking and despite
`ignore_missing_imports` set to `True`:

    tests/test_ipython.py:3: error: Library stubs not installed for "mock" (or incompatible with Python 3.9)
    tests/test_ipython.py:3: note: Hint: "python3 -m pip install types-mock"
    tests/test_ipython.py:3: note: (or run "mypy --install-types" to install all missing stub packages)
    tests/test_ipython.py:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
    tests/test_main.py:7: error: Library stubs not installed for "mock" (or incompatible with Python 3.9)
    Found 2 errors in 2 files (checked 15 source files)

requirements.txt

@@ -7,6 +7,7 @@ pytest-cov
 pytest>=3.9
 sh>=1.09
 tox
+types-mock
 wheel
 twine
 portray

tox.ini

@@ -24,7 +24,8 @@ commands = coverage run --parallel -m pytest {posargs}
 skip_install = true
 deps =
   flake8
-  mypy<0.900
+  mypy
+  types-mock
 commands =
   flake8 src tests
   mypy --python-version=3.9 src tests