Fix mypy issues following the update to mypy 0.901

$ tox -e types
nxdrive/constants.py:6: error: Library stubs not installed for "requests.exceptions" (or incompatible with Python 3.9)
nxdrive/utils.py:1095: error: Library stubs not installed for "requests" (or incompatible with Python 3.9)
nxdrive/utils.py:1096: error: Library stubs not installed for "requests.exceptions" (or incompatible with Python 3.9)
nxdrive/client/proxy.py:4: error: Library stubs not installed for "requests" (or incompatible with Python 3.9)
nxdrive/client/proxy.py:4: note: Hint: "python3 -m pip install types-requests"
nxdrive/client/proxy.py:4: note: (or run "mypy --install-types" to install all missing stub packages)
nxdrive/client/proxy.py:4: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
nxdrive/objects.py:11: error: Library stubs not installed for "dateutil" (or incompatible with Python 3.9)
nxdrive/objects.py:12: error: Library stubs not installed for "dateutil.tz" (or incompatible with Python 3.9)
nxdrive/dao/base.py:23: error: Signature of "execute" incompatible with supertype "Cursor"
nxdrive/dao/base.py:28: error: Too many arguments for "execute" of "Cursor"
nxdrive/updater/base.py:8: error: Library stubs not installed for "requests" (or incompatible with Python 3.9)
nxdrive/updater/base.py:9: error: Library stubs not installed for "yaml" (or incompatible with Python 3.9)
nxdrive/updater/base.py:9: note: Hint: "python3 -m pip install types-PyYAML"
nxdrive/engine/tracker.py:9: error: Library stubs not installed for "requests" (or incompatible with Python 3.9)
nxdrive/client/remote_client.py:23: error: Library stubs not installed for "requests" (or incompatible with Python 3.9)
nxdrive/direct_edit.py:17: error: Library stubs not installed for "requests" (or incompatible with Python 3.9)
nxdrive/engine/queue_manager.py:254: error: unused "type: ignore" comment
nxdrive/engine/engine.py:16: error: Library stubs not installed for "requests" (or incompatible with Python 3.9)
nxdrive/manager.py:15: error: Library stubs not installed for "requests" (or incompatible with Python 3.9)
nxdrive/gui/view.py:4: error: Library stubs not installed for "dateutil.tz" (or incompatible with Python 3.9)
nxdrive/gui/view.py:4: note: Hint: "python3 -m pip install types-python-dateutil"
nxdrive/gui/view.py:499: error: Argument 1 to "format_datetime" of "Translator" has incompatible type "Optional[datetime]"; expected "datetime"
nxdrive/gui/view.py:510: error: Argument 1 to "format_datetime" of "Translator" has incompatible type "Optional[datetime]"; expected "datetime"
nxdrive/gui/view.py:629: error: Argument 1 to "format_datetime" of "Translator" has incompatible type "Optional[datetime]"; expected "datetime"
nxdrive/gui/view.py:640: error: Argument 1 to "format_datetime" of "Translator" has incompatible type "Optional[datetime]"; expected "datetime"
nxdrive/gui/api.py:10: error: Library stubs not installed for "requests" (or incompatible with Python 3.9)
Found 22 errors in 14 files (checked 100 source files)