Fix type errors caused by external libraries

[senofsky]

The following is a snippet of received type errors:

face_mask_detector/lib.py:4: error: Cannot find implementation or library stub for module named 'cv2'
face_mask_detector/lib.py:6: error: Cannot find implementation or library stub for module named 'imutils'
face_mask_detector/lib.py:7: error: Skipping analyzing 'numpy': found module but no type hints or library stubs
face_mask_detector/lib.py:11: error: Cannot find implementation or library stub for module named 'imutils.video'
face_mask_detector/lib.py:12: error: Cannot find implementation or library stub for module named 'tensorflow.keras.applications.mobilenet_v2'
face_mask_detector/lib.py:13: error: Cannot find implementation or library stub for module named 'tensorflow.keras.preprocessing.image'
face_mask_detector/lib.py:14: error: Cannot find implementation or library stub for module named 'tensorflow.keras.models'

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