Enforce type annotations via mypy --strict

Enforcement added via:
- The repo's Makefile (`make check-mypy` & `make lint`)
- The repo's main GitHub Action

.github/workflows/tests.yml

@@ -35,6 +35,9 @@ jobs:
     - name: Validate against flake8
       run: python -m flake8 pdfplumber tests
 
+    - name: Check type annotations via mypy
+      run: python -m mypy --strict pdfplumber
+
   test:
     needs: lint
     runs-on: ubuntu-latest

Makefile

@@ -17,13 +17,16 @@ check-black:
 	${PYTHON} -m black --check pdfplumber tests
 
 check-isort:
-	${PYTHON} -m isort --check-only pdfplumber tests
+	${PYTHON} -m isort --profile black --check-only pdfplumber tests
 
 check-flake:
 	${PYTHON} -m flake8 pdfplumber tests
 
-lint: check-flake check-black check-isort
+check-mypy:
+	${PYTHON} -m mypy --strict pdfplumber
+
+lint: check-flake check-mypy check-black check-isort
 
 format:
 	${PYTHON} -m black pdfplumber tests
-	${PYTHON} -m isort pdfplumber tests
+	${PYTHON} -m isort --profile black pdfplumber tests

requirements-dev.txt

@@ -4,3 +4,7 @@ pytest-parallel==0.1.1
 flake8==4.0.1
 black==22.3.0
 isort==5.10.1
+pandas==1.4.2
+mypy==0.942
+pandas-stubs==1.2.0.58
+types-Pillow==9.0.14