Build docs with Python 3.7 on conda

[takluyver]

This might fix some failures to build on RTD, where Sphinx is failing to import something from the typing module.

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/jupyter-client/conda/latest/bin/sphinx-build", line 6, in <module>
    from sphinx.cmd.build import main
  File "/home/docs/checkouts/readthedocs.org/user_builds/jupyter-client/conda/latest/lib/python3.5/site-packages/sphinx/cmd/build.py", line 22, in <module>
    from sphinx.application import Sphinx
  File "/home/docs/checkouts/readthedocs.org/user_builds/jupyter-client/conda/latest/lib/python3.5/site-packages/sphinx/application.py", line 34, in <module>
    from sphinx.project import Project
  File "/home/docs/checkouts/readthedocs.org/user_builds/jupyter-client/conda/latest/lib/python3.5/site-packages/sphinx/project.py", line 12, in <module>
    from typing import TYPE_CHECKING
ImportError: cannot import name 'TYPE_CHECKING'

[takluyver]

Merging to see if it works; this doesn't affect anything besides building docs.

[takluyver]

This did fix the docs builds.