Issue 787: Replace OG Python string formatting with f-strings

[joedougherty]

Description of changes

This PR aims to address issue https://github.com/rueckstiess/mtools/issues/787

I've replaced the overwhelming majority of .format() instances with equivalent f-strings.

Remaining .format()s:

• mtools/test/test_mloginfo.py (line 100)
• mtools/util/print_table.py (line 28)
• mtools/mlogfilter/mlogfilter.py (line 171)

In these cases it was much less obvious to me how to rework the code to use f-strings, so I've left them in their prior state.

Testing

I ran the included test suite (via tox) to ensure no regressions occurred. All tests pass for:

• Python 3.6
• Python 3.7
• Python 3.8
• Python 3.9

tox output for all four Python environments can be found here: https://gist.github.com/joedougherty/3d7dec2073811a0490bb1d2a53cff952

Should there be Additional Test Coverage?

Curious to know if you think the test suite ought to be extended for better coverage.

As stated above, these changes have been tested with the existing test suite, but may benefit from closer treatment.

Please let me know if you think this is the case.

O/S testing:	O/S	Version(s)
macOS	10.15.7

No testing was done on Windows or Linux.

[stennie]

@joedougherty Thanks for the PR! Apologies for taking a loooong while to review & merge :)

Cheers, Stennie