Update minimum supported Python to 3.8 (#189)

This ought to be possible now that Gecko CI is all using 3.8+.

Author: whimboo

rfcs/python_38.md

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+# RFC 189: Bump minimum supported Python to 3.8
+
+## Summary
+
+Increase the minimum supported Python version to 3.8.
+
+## Details
+
+Currently we support Python 3.7+. However 3.7 was EOL in June 2023,
+and the Python ecosystem has generally moved away from offering
+3.7 support. Practically this means that we have been unable to update
+many dependencies which depend on 3.8+.
+
+Previously the blocker for updates was the downstream integration with
+Gecko's CI which still used Python 3.7. This has recently been updated
+to use [3.8](https://bugzilla.mozilla.org/show_bug.cgi?id=1843209),
+which unblocks updating to that version.
+
+Practically this requires several manual changes:
+
+* Change the CI jobs running under Python 3.7 to instead use 3.8.
+* Update any outdated vendored dependencies that are being held back
+   due to lack of 3.7 support.
+* Retrigger CI runs for dependabot PRs. Hopefully this allow many
+   dependencies to be updated, although there may be some for which
+   the latest versions require 3.9+. In these cases we should manually
+   try to update to the latest 3.8 compatible release.
+
+## Risks
+
+3.8 itself is near the end of its support period (October 2024), so there
+is some risk that we end up in a similar situation again soon, with libraries
+requiring 3.9+. However given the requirement to support vendors, and
+the fact that Gecko depends on 3.8, bumping the minimum version
+further is not possible at this time.
+
+Some unknown external users may depend on 3.7 support. This is
+hopefully reasonably unlikely as they would both need to be using wpt
+in a large system where Python updates are challenging, and also not
+be participating in the RFC process. However if this does happen we
+will need to work with the external user to determine a viable path
+forward.