Add test for fromisoformat

The documented contract of this function is that it is the inverse of
`datetime.isoformat()`.

See GH HypothesisWorks/hypothesis#1401, HypothesisWorks/hypothesis#69
and to a lesser extent HypothesisWorks/hypothesis#2273 for background on
why I have set max_examples so high.

tests/test_datetime.py

@@ -1,6 +1,25 @@
 import unittest
+from datetime import datetime, timezone
+
+from hypothesis import given, settings, strategies as st
+from hypothesis.extra.dateutil import timezones as dateutil_timezones
+
+TIME_ZONES_STRATEGY = st.one_of(
+    st.sampled_from([None, timezone.utc]), dateutil_timezones()
+)
 
 
 class TestDatetime(unittest.TestCase):
     # TODO: https://docs.python.org/3/library/datetime.html
-    pass
+
+    # The search space here is large and as of January 2020 the sampling
+    # functions for characters() and datetimes() are not biased towards
+    # "interesting" examples, so it's worth throwing some extra cycles at this.
+    @settings(max_examples=2500)
+    @given(dt=st.datetimes(timezones=TIME_ZONES_STRATEGY), sep=st.characters())
+    def test_fromisoformat_auto(self, dt, sep):
+        """Test isoformat with timespec="auto"."""
+        dtstr = dt.isoformat(sep=sep, timespec="auto")
+        dt_rt = datetime.fromisoformat(dtstr)
+
+        self.assertEqual(dt, dt_rt)