Clean up python 3.7 deprecation warnings

[jaswilli]

Running the test suite under python 3.7 produces a bunch of deprecation warnings:

/Users/jtw/src/globus/globus-sdk-python/.venv/lib/python3.7/site-packages/jwt/api_jwt.py:4: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  from collections import Iterable, Mapping
/Users/jtw/src/globus/globus-sdk-python/.venv/lib/python3.7/site-packages/jwt/api_jwt.py:4: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  from collections import Iterable, Mapping

/Users/jtw/src/globus/globus-sdk-python/tests/unit/test_config.py:32: DeprecationWarning: This method will be removed in future versions.  Use 'parser.read_file()' instead.
  cfgparser._parser.readfp(configdata)

/Users/jtw/src/globus/globus-sdk-python/tests/unit/test_config.py:32: DeprecationWarning: This method will be removed in future versions.  Use 'parser.read_file()' instead.
  cfgparser._parser.readfp(configdata)

/Users/jtw/src/globus/globus-sdk-python/tests/unit/test_config.py:32: DeprecationWarning: This method will be removed in future versions.  Use 'parser.read_file()' instead.
  cfgparser._parser.readfp(configdata)

/Users/jtw/src/globus/globus-sdk-python/tests/unit/test_config.py:32: DeprecationWarning: This method will be removed in future versions.  Use 'parser.read_file()' instead.
  cfgparser._parser.readfp(configdata)

/Users/jtw/src/globus/globus-sdk-python/tests/unit/test_config.py:32: DeprecationWarning: This method will be removed in future versions.  Use 'parser.read_file()' instead.
  cfgparser._parser.readfp(configdata)

/Users/jtw/src/globus/globus-sdk-python/tests/unit/test_config.py:32: DeprecationWarning: This method will be removed in future versions.  Use 'parser.read_file()' instead.
  cfgparser._parser.readfp(configdata)

/Users/jtw/src/globus/globus-sdk-python/globus_sdk/authorizers/renewing.py:60: DeprecationWarning: The 'warn' method is deprecated, use 'warning' instead
  ("Initializing a RenewingAuthorizer with an "

/Users/jtw/src/globus/globus-sdk-python/globus_sdk/response.py:110: DeprecationWarning: The 'warn' method is deprecated, use 'warning' instead
  logger.warn(('GlobusHTTPResponse.data is null when body is not '
/Users/jtw/src/globus/globus-sdk-python/globus_sdk/response.py:110: DeprecationWarning: The 'warn' method is deprecated, use 'warning' instead
  logger.warn(('GlobusHTTPResponse.data is null when body is not '

-- Docs: https://docs.pytest.org/en/latest/warnings.html