Use setuptools_scm

This allows us to remove the ImportError hack from __init__.py.
It also simplifies how the package is versioned - deriving it solely
from the repository tag.
Both version_info and __version__ have been removed with this change.
Given how these are a unofficial-standard at best, I've found it safer to
always use pkg_resources to determine the version when I need to.

docs/conf.py

@@ -1,4 +1,4 @@
-from sprockets.mixins.mediatype import __version__
+import pkg_resources
 
 needs_sphinx = '1.0'
 extensions = ['sphinx.ext.autodoc',
@@ -8,7 +8,7 @@ extensions = ['sphinx.ext.autodoc',
 master_doc = 'index'
 project = 'sprockets.mixins.mediatype'
 copyright = '2015-2016, AWeber Communications'
-release = __version__
+release = pkg_resources.get_distribution('sprockets.mixins.mediatype').version
 version = '.'.join(release.split('.')[0:1])
 
 html_style = 'custom.css'

setup.py

@@ -4,8 +4,6 @@
 import os
 import setuptools
 
-from sprockets.mixins import mediatype
-
 
 def read_requirements(file_name):
     requirements = []
@@ -30,7 +28,6 @@ tests_require = read_requirements('testing.txt')
 
 setuptools.setup(
     name='sprockets.mixins.mediatype',
-    version=mediatype.__version__,
     description='A mixin for reporting handling content-type/accept headers',
     long_description='\n' + open('README.rst').read(),
     url='https://github.com/sprockets/sprockets.mixins.media_type',
@@ -61,6 +58,8 @@ setuptools.setup(
     extras_require={
         'msgpack': ['u-msgpack-python>=2.5.0,<3']
     },
+    setup_requires=['setuptools_scm'],
+    use_scm_version=True,
     namespace_packages=['sprockets', 'sprockets.mixins'],
     test_suite='nose.collector',
     python_requires='>=3.5',

sprockets/mixins/mediatype/__init__.py

@@ -3,28 +3,11 @@ sprockets.mixins.media_type
 
 """
 
-try:
-    from .content import (ContentMixin, ContentSettings,
-                          add_binary_content_type, add_text_content_type,
-                          set_default_content_type)
-
-except ImportError as error:  # pragma no cover
-    def _error_closure(*args, **kwargs):
-        raise error
-
-    class ErrorClosureClass:
-        def __init__(self, *args, **kwargs):
-            raise error
-
-    ContentMixin = ErrorClosureClass
-    ContentSettings = ErrorClosureClass
-    add_binary_content_type = _error_closure
-    add_text_content_type = _error_closure
-    set_default_content_type = _error_closure
+from .content import (ContentMixin, ContentSettings,
+                      add_binary_content_type, add_text_content_type,
+                      set_default_content_type)
 
 
-version_info = (2, 2, 2)
-__version__ = '.'.join(str(v) for v in version_info)
 __all__ = ['ContentMixin', 'ContentSettings', 'add_binary_content_type',
            'add_text_content_type', 'set_default_content_type',
            'version_info', '__version__']