mypy: move configuration to pyproject.toml (#24802)

This moves our `mypy` configuration from `.mypy.ini` to `.pyproject.toml`
and increases the minimum `mypy` version in the tests.

- [x] move `mypy` configuration to `pyproject.toml`
- [x] remove `.mypy.ini`
- [x] ensure that `mypy` version .900 or higher is used in tests

.github/workflows/unit_tests.yaml

@@ -39,7 +39,7 @@ jobs:
         python-version: 3.9
     - name: Install Python packages
       run: |
-        pip install --upgrade pip six setuptools flake8 isort>=4.3.5 mypy>=0.800 black types-six
+        pip install --upgrade pip six setuptools flake8 isort>=4.3.5 mypy>=0.900 black types-six
     - name: Setup git configuration
       run: |
         # Need this for the git tests to succeed.
@@ -370,7 +370,7 @@ jobs:
       run: |
           pip install --upgrade pip six setuptools
           pip install --upgrade codecov coverage
-          pip install --upgrade flake8 isort>=4.3.5 mypy>=0.800
+          pip install --upgrade flake8 isort>=4.3.5 mypy>=0.900
     - name: Setup Homebrew packages
       run: |
         brew install dash fish gcc gnupg2 kcov