Pin to commit that has some cleanup for new scopes (#63593)

Cleaning up existing code to make the refactoring of Hubs and Scopes
easier. (See https://github.com/getsentry/rfcs/pull/122)

If this dogfooding will not raise any issues, we will merge and release
it in the Sentry SDK.

requirements-getsentry-overrides.txt

@@ -7,3 +7,6 @@
 # the format is:
 # # comment explaining what you're doing
 # library-name @ https://github.com/getsentry/<repo>/archive/<40 char sha>.zip
+
+# Dog fooding some cleanup in the Python SDK
+sentry-sdk @ https://github.com/getsentry/sentry-python/archive/a21349077453840e55f69265953de80a00e987ac.zip