Use strptime at import time to avoid a race

Summary:
We've observed funny AttributeErrors and similar in accessing _strptime, which
are likely the result of a known race. This might possibly fix it.

Test Plan: None

Reviewers: benjamin

Reviewed By: benjamin

Subscribers: changesbot, anupc

Differential Revision: https://tails.corp.dropbox.com/D225912

changes/api/validators/datetime.py

@@ -4,6 +4,10 @@
 
 import logging
 
+# We appear to be hitting https://bugs.python.org/issue7980, but by using
+# strptime early on, the race should be avoided.
+datetime.strptime("", "")
+
 
 class ISODatetime(object):
     def __call__(self, value):