Move stubtest CI to GitHub Actions (#4223)

This ensures that the Python version used matches the one used in the
scheduled extraneous stubtest whitelist check.

See also PR #4206.

tests/stubtest_whitelists/linux.txt

@@ -51,3 +51,7 @@ posix.[a-z]+_(param|result)._(asdict|make|replace)
 # Platform differences that cannot be captured by the type system
 fcntl.[A-Z0-9_]+
 os.SCHED_[A-Z_]+
+
+# Loadable SQLite extensions are disabled on GitHub runners
+sqlite3(.dbapi2)?.Connection.enable_load_extension
+sqlite3(.dbapi2)?.Connection.load_extension

tests/stubtest_whitelists/py38.txt

@@ -48,6 +48,7 @@ hmac.new  # Stub is a white lie; see comments in the stub
 http.client.HTTPSConnection.__init__
 http.cookiejar.DefaultCookiePolicy.__init__
 http.server.SimpleHTTPRequestHandler.__init__
+importlib.metadata.DistributionFinder.Context.pattern
 importlib.metadata.EntryPointBase
 inspect.isasyncgenfunction
 inspect.iscoroutinefunction