Modernize the codebase, reformat and improve performance

[bswck]

This PR contributes to #110 and features various improvements, including:

• Modernize.
  • Use only new-style class declarations class T(object): → class T:.
  • Use types from the built-in scope, not the deprecated ones like typing.List, typing.Dict etc. See PEP 585 and typing docs on built-in types aliases.
  • Postpone type annotations evaluation in runtime. See PEP 563.
  • Use PEP 604 X | Y union type syntax.
  • Use PEP 613 explicit type aliases.
  • Use typing.TYPE_CHECKING to separate typing-related imports from the actual runtime requirements.
  • Consistently use a zero-argument form of super() inside methods.
  • Use pathlib. Use Path(...).read_(text|bytes)() and Path(...).write_(text|bytes)() instead of the open() context manager where possible.
  • Use f"{i:x}" instead of hex(i)[2:]. Act analogically for other numeral systems. More information here.
• Reformat.
  • Split all whole-line-width parameter lists into multi-line signatures with trailing commas.
  • Split one-line expressions that exceed max line length to multi-line.
  • Consistently use double-quote strings (advised and default format from black).
• Improve performance.
  • Replace list/dict/set comprehensions/generator expressions with map/filter where applicable.
  • Prefer on-demand tuples to on-demand lists/sets.
  • Use itertools where applicable. Don't iterate over data passed to functions that accept iterables.
  • Use type(X) instead of X.__class__ where the __class__'s descriptor behavior doesn't matter.
  • Don't catch StopIteration when using next(), simply pass a default value as the other argument.
  • Don't call exception constructor when raising it with no message.

Note

To inspect the changes more easily, you might review every commit one-by-one chronologically.

All tests pass on my end.

[bswck]

During the review, a special attention should be paid to statements not covered by tests.

Coverage report before changes: https://smokeshow.helpmanual.io/3u0g2z34522g3a2k6k4u/
Coverage report after changes: https://smokeshow.helpmanual.io/640r041r4j1m2a2y6p1e/

[bswck]

@psrok1 I made this PR before a job interview @ NASK (unsuccessful). I'm curious about your feedback though and whether I should continue with the refactor. It inspired me to invent https://github.com/bswck/autorefine.