Clean mypy cache on each run of stubtest (#562)

Same as was merged into django-stubs:

* typeddjango/django-stubs#2015

Author: intgr

scripts/stubtest.sh

@@ -6,6 +6,9 @@ set -e
 
 export MYPYPATH='.'
 
+# Cleaning existing cache:
+rm -rf .mypy_cache
+
 # TODO: remove `--ignore-positional-only` when ready
 stubtest rest_framework \
     --mypy-config-file mypy.ini \