Decrease Mypy concurrent process count (#1618)

Mypy has issues with running its test suite with many processes
concurrently. This should reduce travis test failures, if not completely
resolve failures. See issue python/mypy#3543

Author: emmatyping

tests/mypy_selftest.py

@@ -19,7 +19,7 @@
         shutil.copytree('stdlib', str(dirpath / 'mypy/typeshed/stdlib'))
         shutil.copytree('third_party', str(dirpath / 'mypy/typeshed/third_party'))
         try:
-            subprocess.run(['./runtests.py'], cwd=str(dirpath / 'mypy'), check=True)
+            subprocess.run(['./runtests.py', '-j12'], cwd=str(dirpath / 'mypy'), check=True)
         except subprocess.CalledProcessError as e:
             print('mypy tests failed', file=sys.stderr)
             sys.exit(e.returncode)