sunpy/sunpy: v0.9.3

Sunpy 0.9.3 (2018-09-12) Bug Fixes Correctly import [~astropy.units.allclose]{.title-ref} based on astropy version. This means that [sunpy.coordinates]{.title-ref} will import when pytest is not installed. (#2702) Raise an error when transforming between HPC and HCC frames if the observer is not the same. (#2725) Do not attempt to save a FITS header comment for a keyword which is not in the header. This prevents an error on saving some maps after the metadata had been modified but not the comments. (#2748) Add support for [HMIMap]{.title-ref} objects as input to [sunpy.instr.aia.aiaprep()]{.title-ref}. (#2749) Improved Documentation Add contribution guidelines for the sunpy example gallery. (#2682) Clean up the docstring for [sunpy.physics.differential_rotation.solar_rotate_coordinate]{.title-ref} to make the example clearer. (#2708