Use tomli in place of unmaintained toml package

[mgorny]

Use the modern tomli TOML parser instead of toml. The latter package is no longer maintained and does not support TOML 1.0.

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Open question: tomli doesn't support Python 2. Do you prefer if I removed Python 2 support entirely, did some conditionals to support toml fallback on Python 2 or perhaps simply ignored missing pyproject.toml config support in Python 2 and skipped respective tests?

[codecov-commenter]

Codecov Report

Base: 97.91% // Head: 97.87% // Decreases project coverage by -0.04% :warning:

Coverage data is based on head (d083694) compared to base (02bcfbf). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #629 +/- ## ========================================== - Coverage 97.91% 97.87% -0.05% ========================================== Files 1 1 Lines 2399 2399 ========================================== - Hits 2349 2348 -1 - Misses 50 51 +1 ``` | [Impacted Files](https://codecov.io/gh/hhatto/autopep8/pull/629?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hideo+Hattori) | Coverage Δ | | |---|---|---| | [autopep8.py](https://codecov.io/gh/hhatto/autopep8/pull/629/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hideo+Hattori#diff-YXV0b3BlcDgucHk=) | `97.87% <100.00%> (-0.05%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hideo+Hattori). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hideo+Hattori)

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[mgorny]

Ping.

[hhatto]

@mgorny Sorry for late reply.

merged this pr. Thanks for your contribution 👍

[mgorny]

Thanks!