drop support for Python 3.7

[davidhewitt]

Cryptography dropped 3.7 support almost a year ago, I think it's long overdue that we can remove support too.

pyca/cryptography#12925

[codspeed-hq]

Merging this PR will not alter performance

✅ 105 untouched benchmarks
⏩ 1 skipped benchmark¹

---

_{Comparing davidhewitt:drop-3.7 (d4890c0) with main (603a69a)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[Icxolu]

Nice, great to see some many simplifications 🎉

[bschoenmaeckers]

Amazing!