Release v7.2.1

- Bump version.
- Update spec and stone.
- Make update_version.sh more portable.
- Update .gitignore.

.gitignore

@@ -1,4 +1,5 @@
 __pycache__/
+.cache/
 *.egg
 *.egg-info/
 *.pyc

dropbox/dropbox.py

@@ -4,7 +4,7 @@
     'create_session',
 ]
 
-__version__ = '7.2.0'
+__version__ = '7.2.1'
 
 import contextlib
 import json

dropbox/stone_base.py

@@ -15,12 +15,16 @@
     # This makes testing this file directly (outside of a package) easier.
     import stone_validators as bv  # type: ignore
 
+_MYPY = False
+if _MYPY:
+    import typing  # noqa: F401 # pylint: disable=import-error,unused-import,useless-suppression
 
-class Union(object):
 
+class Union(object):
     # TODO(kelkabany): Possible optimization is to remove _value if a
     # union is composed of only symbols.
     __slots__ = ['_tag', '_value']
+    _tagmap = {}  # type: typing.Dict[typing.Text, bv.Validator]
 
     def __init__(self, tag, value=None):
         assert tag in self._tagmap, 'Invalid tag %r.' % tag