Switch to versioneer to simplify version handling

Author: aiudirog

Diff for: .gitattributes

@@ -0,0 +1 @@
+httpx_gssapi/_version.py export-subst

Diff for: MANIFEST.in

@@ -3,3 +3,6 @@ include README.rst
 include LICENSE
 include HISTORY.rst
 include AUTHORS
+include versioneer.py
+include httpx_gssapi/_version.py
+include test_httpx_gssapi.py

Diff for: httpx_gssapi/__init__.py

@@ -21,12 +21,15 @@
     'OPTIONAL',
     'DISABLED',
 )
-__version__ = '1.0.0'
 
 import logging
 
 from .gssapi_ import HTTPSPNEGOAuth, REQUIRED, OPTIONAL, DISABLED
 from .exceptions import MutualAuthenticationError
 from .compat import HTTPKerberosAuth
 
+from ._version import get_versions
+__version__ = get_versions()['version']
+del get_versions
+
 logging.getLogger(__name__).addHandler(logging.NullHandler())