Fix mypy issues

Author: rhpvorderman

src/isal/__init__.py

@@ -17,11 +17,15 @@
 # LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
+from typing import Optional
 
 try:
-    from ._isal import ISAL_MAJOR_VERSION, ISAL_MINOR_VERSION, \
-        ISAL_PATCH_VERSION, ISAL_VERSION
-except ImportError:  # Not available on windows
+    from . import _isal
+    ISAL_MAJOR_VERSION: Optional[int] = _isal.ISAL_MAJOR_VERSION
+    ISAL_MINOR_VERSION: Optional[int] = _isal.ISAL_MINOR_VERSION
+    ISAL_PATCH_VERSION: Optional[int] = _isal.ISAL_PATCH_VERSION
+    ISAL_VERSION: Optional[str] = _isal.ISAL_VERSION
+except ImportError:  # isa-l.h not available on windows
     ISAL_MAJOR_VERSION = None
     ISAL_MINOR_VERSION = None
     ISAL_PATCH_VERSION = None