Add __deepcopy__ method to TomlTz class. Fix uiri#308 (uiri#309)

Without this method, the dict that results from parsing a datetime
with a timezone cannot be deep-copied using the stdlib
`copy.deepcopy()` function.

tests/test_api.py

@@ -271,3 +271,12 @@ def test_comment_preserve_decoder_encoder():
                    encoder=toml.TomlPreserveCommentEncoder())
 
     assert len(s) == len(test_str) and sorted(test_str) == sorted(s)
+
+
+def test_deepcopy_timezone():
+    import copy
+
+    o = toml.loads("dob = 1979-05-24T07:32:00-08:00")
+    o2 = copy.deepcopy(o)
+    assert o2["dob"] == o["dob"]
+    assert o2["dob"] is not o["dob"]

toml/tz.py

@@ -11,6 +11,9 @@ def __init__(self, toml_offset):
         self._hours = int(self._raw_offset[1:3])
         self._minutes = int(self._raw_offset[4:6])
 
+    def __deepcopy__(self, memo):
+        return self.__class__(self._raw_offset)
+
     def tzname(self, dt):
         return "UTC" + self._raw_offset