Importing `uplink_protobuf` before `uplink` causes ImportError

[prkumar]

Importing uplink_protobuf before uplink raises the following exception:

>>> import uplink_protobuf
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/site-packages/uplink_protobuf/__init__.py", line 2, in <module>
    from uplink_protobuf import json_options
  File "/usr/local/lib/python2.7/site-packages/uplink_protobuf/json_options.py", line 3, in <module>
    from uplink import decorators
  File "/usr/local/lib/python2.7/site-packages/uplink/__init__.py", line 91, in <module>
    _load_entry_points()
  File "/usr/local/lib/python2.7/site-packages/uplink/_extras.py", line 40, in load_entry_points
    for entry_point in _iter_entry_points(name)
  File "/usr/local/lib/python2.7/site-packages/uplink/_extras.py", line 40, in <dictcomp>
    for entry_point in _iter_entry_points(name)
  File "/usr/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2318, in load
    return self.resolve()
  File "/usr/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2328, in resolve
    raise ImportError(str(exc))
ImportError: 'module' object has no attribute 'ProtocolBuffersConverter'

Interestingly, after uplink_protobuf fails to import, uplink starts throwing an ImportError on each import.

>>> import uplink
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/site-packages/uplink/__init__.py", line 4, in <module>
    from uplink import returns, types
ImportError: cannot import name returns

Verified on Python 2.7.15