Enable `flake8-future-annotations`

[CoolCat467]

This pull request enables ruff's flake8-future-annotations rule, building off of the formatting changes done in https://github.com/python-trio/trio/pull/2930.

[codecov[bot]]

Codecov Report

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

Comparison is base (b780204) 99.64% compared to head (9d1035b) 99.64%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2932 +/- ## ======================================= Coverage 99.64% 99.64% ======================================= Files 116 116 Lines 17501 17503 +2 Branches 3148 3148 ======================================= + Hits 17439 17441 +2 Misses 43 43 Partials 19 19 ``` | [Files](https://app.codecov.io/gh/python-trio/trio/pull/2932?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio) | Coverage Δ | | |---|---|---| | [src/trio/\_subprocess\_platform/\_\_init\_\_.py](https://app.codecov.io/gh/python-trio/trio/pull/2932?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX3N1YnByb2Nlc3NfcGxhdGZvcm0vX19pbml0X18ucHk=) | `100.00% <100.00%> (ø)` | | | [src/trio/\_tests/test\_file\_io.py](https://app.codecov.io/gh/python-trio/trio/pull/2932?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX3Rlc3RzL3Rlc3RfZmlsZV9pby5weQ==) | `100.00% <100.00%> (ø)` | | | [src/trio/socket.py](https://app.codecov.io/gh/python-trio/trio/pull/2932?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vc29ja2V0LnB5) | `100.00% <100.00%> (ø)` | |