Add test that proves datetime precision consistency (#60)

* Add test that proves datetime precision consistency
See #22 (comment)

* Fix linting

tests/test_misc.py

@@ -2,6 +2,8 @@
 from pathlib import Path
 from types import ModuleType
 
+import pytest
+
 import rtoml
 
 
@@ -17,3 +19,17 @@ def test_example():
 def test_version():
     assert isinstance(rtoml.__version__, str)
     print('rtoml __version__:', rtoml.__version__)
+
+
+@pytest.mark.parametrize(
+    'dt',
+    [
+        # passes, for 6 significant subsecond digits
+        '2020-05-25T12:00:01.123456',
+        # previously failed because last subsecond digit is not dumped to string
+        # and load parsing fails with only 5 fractional digits
+        '2020-05-25T12:00:01.123450',
+    ],
+)
+def test_datetime_precision(dt):
+    assert rtoml.loads(rtoml.dumps({'a': dt})) == {'a': dt}