Drop Python 3.7 leftovers

[mdmintz]

Type of changes

• [X] Documentation / docstrings
• [X] Tests
• [X] Other

Description

Some Python 3.7 leftovers remained when support for Python 3.7 was dropped in https://github.com/Textualize/rich/releases/tag/v13.9.0.

This completes the drop. Eg. PyPI still shows support for 3.7.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.25%. Comparing base (9c74f03) to head (0a549ce). Report is 83 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3514 +/- ## ======================================= Coverage 98.25% 98.25% ======================================= Files 74 74 Lines 8072 8098 +26 ======================================= + Hits 7931 7957 +26 Misses 141 141 ``` | [Flag](https://app.codecov.io/gh/Textualize/rich/pull/3514/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/Textualize/rich/pull/3514/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `98.25% <ø> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#carryforward-flags-in-the-pull-request-comment) to find out more.

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[willmcgugan]

Thanks