Use local pytest-cache loader

Additional code is required to properly initalize pytest-cache when it's installed into a local egg in the current working directory.

Fix from here:

    https://bitbucket.org/hpk42/pytest-cache/issue/8/pytest-cache-plugin-not-loading-when-using

Without this, I get:
.....
running build_ext
usage: setup.py [options] [file_or_dir] [file_or_dir] [...]
setup.py: error: unrecognized arguments: --clearcache

setup.py

@@ -38,6 +38,9 @@ def finalize_options(self):
 
     def run_tests(self):
         import pytest
+        import _pytest.config
+        pm = _pytest.config.get_plugin_manager()
+        pm.consider_setuptools_entrypoints()
         errno = pytest.main(self.test_args)
         sys.exit(errno)