Add requirements for documentation build

.readthedocs.yml

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+# .readthedocs.yml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+  configuration: docs/conf.py
+
+# Optionally build your docs in additional formats such as PDF and ePub
+formats: all
+
+# Optionally set the version of Python and requirements required to build your docs
+python:
+  version: 3.7
+  install:
+    - requirements: docs/requirements.txt

docs/requirements.txt

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+alabaster==0.7.12
+Babel==2.7.0
+certifi==2019.11.28
+chardet==3.0.4
+docutils==0.15.2
+idna==2.8
+imagesize==1.1.0
+Jinja2==2.10.3
+MarkupSafe==1.1.1
+packaging==19.2
+Pygments==2.5.2
+pyparsing==2.4.5
+pytz==2019.3
+requests==2.22.0
+six==1.13.0
+snowballstemmer==2.0.0
+Sphinx==2.2.2
+sphinx-rtd-theme==0.4.3
+sphinxcontrib-applehelp==1.0.1
+sphinxcontrib-devhelp==1.0.1
+sphinxcontrib-htmlhelp==1.0.2
+sphinxcontrib-jsmath==1.0.1
+sphinxcontrib-qthelp==1.0.2
+sphinxcontrib-serializinghtml==1.1.3
+urllib3==1.25.7