Use parallel checking in self check (#21221)

Let's start dogfooding parallel type checking. This is about 3x faster
for a cold run than sequential on my mac laptop (not compiled).

Author: JukkaL

mypy_self_check.ini

@@ -23,3 +23,4 @@ show_traceback = True
 
 # Miscellaneous
 sqlite_cache = True
+num_workers = 4