Update minimum supported Python to 3.7 (#134)

This ought to be possible now that Gecko CI is all using 3.7+.

Author: jgraham

rfcs/python_37.md

@@ -0,0 +1,43 @@
+# RFC 134: Bump minimum supported Python to 3.7
+
+## Summary
+
+Increase the minimum supported Python version to 3.7.
+
+## Details
+
+Currently we support Python 3.6+. However 3.6 was EOL in December
+2021, and the Python ecosystem has generally moved away from offering
+3.6 support. Practically this means that we have been unable to update
+many dependencies which depend on 3.7+.
+
+Previously the blocker for updates was the downstream integration with
+Gecko's CI which still used Python 3.6. This has recently been updated
+to use [3.7](https://bugzilla.mozilla.org/show_bug.cgi?id=1734402),
+which unblocks updating to that version.
+
+Practically this requires several manual changes:
+
+ * Change the CI jobs running under Python 3.6 to instead use 3.7.
+ * Update any outdated vendored dependencies that are being held back
+   due to lack of 3.6 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 3.8+. In these cases we should manually
+   try to update to the latest 3.7 compatible release.
+
+
+## Risks
+
+3.7 itself is near the end of its support period, so there is some
+risk that we end up in a similar situation again soon, with libraries
+requiring 3.8+. However given the requirement to support vendors, and
+the fact that Gecko depends on 3.7, bumping the minimum version
+further is not possible at this time.
+
+Some unknown external users may depend on 3.6 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.