feat: add 'py.typed' declaration

.github/workflows/presubmit.yml

@@ -7,6 +7,22 @@ on:
 
 jobs:
 
+  mypy:
+    runs-on: ubuntu-20.04
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python
+      uses: actions/setup-python@v2
+      with:
+        python-version: "3.6"
+    - name: Install nox
+      run: |
+        python -m ensurepip
+        python -m pip install nox
+    - name: Run mypy
+      run: |
+        nox -s mypy
+
   build-wheels-linux:
     name: ${{ matrix.os }}
     strategy:

mypy.ini

@@ -0,0 +1,3 @@
+[mypy]
+python_version = 3.6
+exclude = tests/unit/resources/

noxfile.py

@@ -47,3 +47,15 @@ def check(session):
     # Run py.test against the unit tests.
     session.run("py.test", "tests")
     session.run("python", f"{HERE}/scripts/check_crc32c_extension.py", *session.posargs)
+
+
+@nox.session(python="3.6")
+def mypy(session):
+    """Verify type hints are mypy compatible."""
+    session.install("-e", ".")
+    session.install(
+        "mypy",
+        "types-mock",
+        "types-setuptools",
+    )
+    session.run("mypy", "src/google_crc32c/", "tests/")

src/google_crc32c/__init__.py

@@ -25,7 +25,7 @@
     from google_crc32c import cext as impl
     implementation = "c"
 except ImportError as exc:
-    from google_crc32c import python as impl
+    from google_crc32c import python as impl  # type: ignore
     warnings.warn(_SLOW_CRC32C_WARNING, RuntimeWarning)
     implementation = "python"
 

src/google_crc32c/cext.py

@@ -16,9 +16,9 @@
 
 # NOTE: ``__config__`` **must** be the first import because it (may)
 #       modify the search path used to locate shared libraries.
-import google_crc32c.__config__
-from google_crc32c._crc32c import extend
-from google_crc32c._crc32c import value
+import google_crc32c.__config__  # type: ignore
+from google_crc32c._crc32c import extend  # type: ignore
+from google_crc32c._crc32c import value  # type: ignore
 from google_crc32c._checksum import CommonChecksum
 
 

src/google_crc32c/py.typed

@@ -0,0 +1,2 @@
+# Marker file for PEP 561.
+# The google_crc32c package uses inline types.

tests/test___init__.py

@@ -15,7 +15,7 @@
 import itertools
 from unittest import mock
 
-import pytest
+import pytest  # type: ignore
 
 import google_crc32c