add mypy_selftest.py (#1102)

* add mypy_selftest.py
* add selftest to Travis

tests/mypy_selftest.py

@@ -0,0 +1,27 @@
+#!/usr/bin/env python3
+"""Script to run mypy's test suite against this version of typeshed."""
+
+from pathlib import Path
+import shutil
+import subprocess
+import sys
+import tempfile
+
+
+if __name__ == '__main__':
+    with tempfile.TemporaryDirectory() as tempdir:
+        dirpath = Path(tempdir)
+        subprocess.run(['git', 'clone', '--depth', '1', 'git://github.com/python/mypy',
+                        str(dirpath / 'mypy')], check=True)
+        subprocess.run([sys.executable, '-m', 'pip', 'install', '-U', '-r',
+                        str(dirpath / 'mypy/test-requirements.txt')], check=True)
+        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)
+        except subprocess.CalledProcessError as e:
+            print('mypy tests failed', file=sys.stderr)
+            sys.exit(e.returncode)
+        else:
+            print('mypy tests succeeded', file=sys.stderr)
+            sys.exit(0)