Fix mypy test error · evhub/coconut@5f8bc6c

Process completed with exit code 2.

CPython version >=3.6.0 <3.7.0-0 is not supported by actions/setup-python. This probably means you are using a deprecated version. If this is not the case, you may suppress the warning setting input "allow-build" to either "info" or "allow".

Specific version 3.6.15 is no longer officially supported. This action will not break support intentionally but no effort will be spent in supporting it.