Use a wrapper script to bypass check-manifest if not under git

Related to comment in #2051

scripts/check-manifest.py

@@ -0,0 +1,21 @@
+"""
+Script used by tox.ini to check the manifest file if we are under version control, or skip the
+check altogether if not.
+
+"check-manifest" will needs a vcs to work, which is not available when testing the package
+instead of the source code (with ``devpi test`` for example).
+"""
+
+from __future__ import print_function
+
+import os
+import subprocess
+import sys
+
+
+if os.path.isdir('.git'):
+    sys.exit(subprocess.call('check-manifest', shell=True))
+else:
+    print('No .git directory found, skipping checking the manifest file')
+    sys.exit(0)
+