Stop auto-converting timezones to UTC and stripping timezone info (#1581)

* Added timezones future flag
* Don't convert datetimes to UTC or strip timezone info during JSON
encoding

Author: jonmmease

Diff for: _plotly_future_/timezones.py

@@ -0,0 +1,5 @@
+from __future__ import absolute_import
+from _plotly_future_ import _future_flags, _assert_plotly_not_imported
+
+_assert_plotly_not_imported()
+_future_flags.add('timezones')

Diff for: _plotly_future_/v4.py

@@ -6,6 +6,7 @@
     remove_deprecations,
     v4_subplots,
     orca_defaults,
+    timezones,
     trace_uids,
 )
 

Diff for: _plotly_utils/utils.py

@@ -3,8 +3,8 @@
 import json as _json
 import sys
 import re
-
 import pytz
+from _plotly_future_ import _future_flags
 
 from _plotly_utils.optional_imports import get_module
 
@@ -104,7 +104,9 @@ def default(self, obj):
             self.encode_as_sage,
             self.encode_as_numpy,
             self.encode_as_pandas,
-            self.encode_as_datetime,
+            (self.encode_as_datetime_v4
+             if 'timezones' in _future_flags
+             else self.encode_as_datetime),
             self.encode_as_date,
             self.encode_as_list,  # because some values have `tolist` do last.
             self.encode_as_decimal
@@ -170,6 +172,14 @@ def encode_as_numpy(obj):
         else:
             raise NotEncodable
 
+    @staticmethod
+    def encode_as_datetime_v4(obj):
+        """Convert datetime objects to iso-format strings"""
+        try:
+            return obj.isoformat()
+        except AttributeError:
+            raise NotEncodable
+
     @staticmethod
     def encode_as_datetime(obj):
         """Attempt to convert to utc-iso time string using datetime methods."""