try using --no-site-packages (#2036)

Fixes #2035.

tests/mypy_test.py

@@ -131,6 +131,7 @@ def main():
             runs += 1
             flags = ['--python-version', '%d.%d' % (major, minor)]
             flags.append('--strict-optional')
+            flags.append('--no-site-packages')
             if args.no_implicit_optional:
                 flags.append('--no-implicit-optional')
             if args.warn_unused_ignores: