Description of problem:
Python 3.7 has deprecated ABCs in the collection module. Instead you have to import ABCs from collections.abs.

Version-Release number of selected component (if applicable):
1.3.0, 1.5.0

How reproducible:
Always

Steps to Reproduce:
1. dnf install python37
2. python3.7 -m venv /tmp/venv
3. /tmp/venv/bin/pip install gssapi
4. /tmp/venv/bin/python -Werror::DeprecationWarning -c "import gssapi"

Actual results:
$ /tmp/venv/bin/python -Werror::DeprecationWarning -c "import gssapi"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/tmp/venv/lib64/python3.7/site-packages/gssapi/__init__.py", line 29, in <module>
    from gssapi.raw.types import NameType, RequirementFlag, AddressType  # noqa
  File "/tmp/venv/lib64/python3.7/site-packages/gssapi/raw/__init__.py", line 50, in <module>
    from gssapi.raw.creds import *  # noqa
  File "gssapi/raw/names.pxd", line 3, in init gssapi.raw.creds
  File "gssapi/raw/names.pyx", line 6, in init gssapi.raw.names
  File "gssapi/raw/misc.pyx", line 1, in init gssapi.raw.misc
  File "gssapi/raw/cython_converters.pyx", line 4, in init gssapi.raw.cython_converters
  File "gssapi/raw/types.pyx", line 109, in init gssapi.raw.types
  File "/usr/lib64/python3.7/collections/__init__.py", line 52, in __getattr__
    DeprecationWarning, stacklevel=2)
DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working


Expected results:
no error

Additional info:
The deprecation warning breaks Custodia's tests.