use Optional[] type hints

[atenni]

For arguments that have a default of None PEP-484 now recommends
using Optional[] (previously these optional types were assumed). It
also recommends type checkers move towards requiring the optional
type to be made explicit, so this commit will future proof for this
behaviour.

See: https://peps.python.org/pep-0484/#union-types

Closes: #419

[pep8speaks]

Hello @atenni! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file webdriver_manager/firefox.py:

Line 17:80: E501 line too long (101 > 79 characters)
Line 18:80: E501 line too long (103 > 79 characters)

• In the file webdriver_manager/microsoft.py:

Line 19:80: E501 line too long (94 > 79 characters)
Line 20:80: E501 line too long (108 > 79 characters)
Line 47:80: E501 line too long (86 > 79 characters)

• In the file webdriver_manager/opera.py:

Line 18:9: E131 continuation line unaligned for hanging indent
Line 19:9: E131 continuation line unaligned for hanging indent

[SergeyPirogov]

@atenni thank you