[docs] add readthedocs configuration

.readthedocs.yaml

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+# .readthedocs.yaml
+# 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: doc/source/conf.py
+
+# Optionally build your docs in additional formats such as PDF
+formats:
+   - pdf
+
+# Optionally set the version of Python and requirements required to build your docs
+python:
+   version: "3.7"
+   install:
+     - requirements: doc/requirements.txt
+     - method: setuptools
+       path: doc

doc/requirements.txt

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+# packages required to build docs (needed by readthedocs)
+numpy==1.19.1
+sphinx==3.5.1
+sympy==1.8
+enscons==0.26
+toml==0.10
+packaging==20.5

doc/setup.py

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+import sys, os
+exit(os.system(f"{sys.executable} -m SCons pySecDec/metadata.py"))