Fix return type for get_token in base.py

[sandrobonazzola]

No description provided.

[sandrobonazzola]

Converting to draft as it fails on pytthon 3.9 with:

==================================== ERRORS ====================================
_____________________ ERROR collecting tests/test_base.py ______________________
tests/test_base.py:14: in <module>
    import did.base
did/base.py:495: in <module>
    token_file_key: str = "token_file") -> str | None:
E   TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'
______________________ ERROR collecting tests/test_cli.py ______________________
tests/test_cli.py:9: in <module>
    import did.base
did/base.py:495: in <module>
    token_file_key: str = "token_file") -> str | None:
E   TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'
_____________________ ERROR collecting tests/test_stats.py _____________________
tests/test_stats.py:3: in <module>
    import did.stats
did/stats.py:8: in <module>
    import did.base
did/base.py:495: in <module>
    token_file_key: str = "token_file") -> str | None:
E   TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'