MNT: Clean up manifest

MANIFEST.in

@@ -1,21 +1,18 @@
 include README.rst
 include CHANGES.rst
+include LICENSE.rst
 
 include ah_bootstrap.py
 include setup.cfg
 include astrowidgets/tests/coveragerc
 
-recursive-include astrowidgets *.pyx *.c *.pxd
 recursive-include docs *
 recursive-include licenses *
-recursive-include cextern *
-recursive-include scripts *
 
 prune build
 prune docs/_build
 prune docs/api
 
-
 # the next few stanzas are for astropy_helpers.  It's derived from the
 # astropy_helpers/MANIFEST.in, but requires additional includes for the actual
 # package directory and egg-info.
@@ -35,5 +32,6 @@ recursive-include astropy_helpers/astropy_helpers/sphinx *
 prune astropy_helpers/build
 prune astropy_helpers/astropy_helpers/tests
 
+# Miscellaneous
 
 global-exclude *.pyc *.o