astropy 3 incompatibilities on RTD

[JarronL]

• pyNRC version: 0.6.2
• Python version: 3.6

readthedocs seems to fail with astropy 3.0.1.

Searching for astropy>=2.0.3
Reading https://pypi.python.org/simple/astropy/
Downloading https://pypi.python.org/packages/b8/6d/5e15b13b33c8b1429446eb9432b876a10ad48655fba61e33500b8ad53dd6/astropy-3.0.1.tar.gz#md5=d54c2571fdb82642fcce27f5a88f33d3
Best match: astropy 3.0.1
Processing astropy-3.0.1.tar.gz
Writing /tmp/easy_install-7dxc5rhx/astropy-3.0.1/setup.cfg
Running astropy-3.0.1/setup.py -q bdist_egg --dist-dir /tmp/easy_install-7dxc5rhx/astropy-3.0.1/egg-dist-tmp-57z6a2ys
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/pynrc/envs/stable/lib/python3.5/site-packages/setuptools/sandbox.py", line 154, in save_modules
    yield saved
  File "/home/docs/checkouts/readthedocs.org/user_builds/pynrc/envs/stable/lib/python3.5/site-packages/setuptools/sandbox.py", line 195, in setup_context
    yield
  File "/home/docs/checkouts/readthedocs.org/user_builds/pynrc/envs/stable/lib/python3.5/site-packages/setuptools/sandbox.py", line 250, in run_setup
    _execfile(setup_script, ns)
  File "/home/docs/checkouts/readthedocs.org/user_builds/pynrc/envs/stable/lib/python3.5/site-packages/setuptools/sandbox.py", line 45, in _execfile
    exec(code, globals, locals)
  File "/tmp/easy_install-7dxc5rhx/astropy-3.0.1/setup.py", line 117, in <module>
    ],
AttributeError: module 'astropy' has no attribute '__minimum_python_version__'

Current work around is to force astropy<3 in setup.py and requirements.txt