docs: Use python 3.7 via conda for readthedocs builds

The typing module in python 3.5 has problems with some type
annotations so we need to build with 3.7. RTD doesn't yet support py37
natively but we can get it with conda following an example at
readthedocs/readthedocs-docker-images#73

Author: bdarnell

docs/conda.yml

@@ -0,0 +1,8 @@
+# TODO: remove when RTD supports python 3.7 without conda
+name: tornado-docs
+dependencies:
+  - python=3.7
+  - pip:
+    - sphinx
+    - sphinx-rtd-theme
+    - Twisted

readthedocs.yml

@@ -0,0 +1,4 @@
+# TODO: remove when RTD supports Python 3.7 without conda.
+# https://github.com/rtfd/readthedocs-docker-images/pull/73
+conda:
+  file: docs/conda.yml