This is likely a wontfix, but old versions of the plugin call any output an error. Newer versions of mypy (0.730 at least) produce output on success which causes a false positive:

=========================== FAILURES ============================ __________________________ mypy cli.py __________________________ Success: no issues found in 1 source file 

The solution is to upgrade to pytest-mypy >= 0.4.0 or downgrade mypy.