Fix mypy issues following the update to mypy 0.770

$ tox -e types
types create: D:\a\nuxeo-drive\nuxeo-drive\.tox\types
types installdeps: -rtools/deps/requirements-pip.txt, -rtools/deps/requirements-types.txt
types installed: mypy==0.770,mypy-extensions==0.4.3,typed-ast==1.4.1,typing-extensions==3.7.4.1
types run-test-pre: PYTHONHASHSEED='48'
types run-test: commands[0] | python -m mypy --platform win32 nxdrive
nxdrive\engine\dao\utils.py:38: error: "iterdump" of "Connection" does not return a value
nxdrive\utils.py:131: error: unused 'type: ignore' comment
nxdrive\client\local\base.py:435: error: unused 'type: ignore' comment
nxdrive\updater\darwin.py:104: error: Argument 1 to "move" has incompatible type "Path"; expected "str"
nxdrive\osi\linux\linux.py:34: error: Item "None" of "Optional[IO[bytes]]" has no attribute "write"
nxdrive\osi\linux\linux.py:35: error: Item "None" of "Optional[IO[bytes]]" has no attribute "close"
nxdrive\osi\darwin\darwin.py:114: error: Item "None" of "Optional[IO[bytes]]" has no attribute "write"
nxdrive\osi\darwin\darwin.py:115: error: Item "None" of "Optional[IO[bytes]]" has no attribute "close"
Found 8 errors in 6 files (checked 71 source files)