types has no attribute StringTypes in Python3. Update this to str

[jamespeacock]

File "/usr/local/lib/python3.6/site-packages/encrypted_fields/fields.py", line 163, in to_python
if value is None or not isinstance(value, types.StringTypes):
AttributeError: module 'types' has no attribute 'StringTypes'

[jonathan-chao-artivest]

please fix this asap. I had to manually change it in site-package and I would really love to avoid that.

There are some pending PRs that have this change already