chore: Bump version to 2.7.3

Note: It looks like the version had been previously bumped to 2.7.2,
though the corresponding dependency has not yet been upgraded in
`edx-platform`.

setup.py

@@ -49,7 +49,7 @@ with open('README.rst') as _f:
 
 setup(
     name='lti-consumer-xblock',
-    version='2.7.2',
+    version='2.7.3',
     description='This XBlock implements the consumer side of the LTI specification.',
     long_description=long_description,
     long_description_content_type='text/x-rst',