Py3 merge onto main

Py3 branch merged onto main branch

Python 3 compatibility

Included and tested Python 3.6+ compatibility and backwards compatible with Python 2.6+

v2.0.1: Minor correction to README.rst

From 009f6c8 (master):
=====================

In README.rst,

(*) Minor update -- correction of a typo

v1.10: Fix gsm2016 in diffuse_radio_sky_model()

From f785a23 (skymod):
=====================

In astroutils/catalog.py,

(*) In function diffuse_radio_sky_model(), pass explicit
initialization parameters in call to GlobalSkyModel2016() of the
PyGSM module
(*) Generate gsm2016 output initially in Rayleigh-Jeans temperature
units and then convert to flux density.