Add basic type stubs (#221) (#215)

* feat: add basic type stubs

* feat: add types for constants

* feat: add type for `MarkdownConverter` class

* ci: add basic job for checking types

* feat: add new constant

* ci: install types as required

* ci: install types package manually

* test: add strict coverage for types

* fix: allow `strip_document` to be `None`

* feat: expand types for MarkdownConverter

* fix: do not use `Unpack` as it requires Python 3.12

* feat: define `MarkdownConverter#convert_soup`

* feat: improve type for `code_language_callback`

* chore: add end-of-file newline

* refactor: use `Union` for now

.github/workflows/python-app.yml

@@ -30,3 +30,22 @@ jobs:
     - name: Build
       run: |
         python -m build -nwsx .
+
+  types:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python 3.8
+      uses: actions/setup-python@v2
+      with:
+        python-version: 3.8
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install --upgrade setuptools setuptools_scm wheel build tox mypy types-beautifulsoup4
+    - name: Check types
+      run: |
+        mypy .
+        mypy --strict tests/types.py