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
eugene2candy · Dec 30, 2018 · e7e31e5 · e7e31e5
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diff --git a/docs/conda.yml b/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
diff --git a/readthedocs.yml b/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