Fix docs building (#3263)

setup.py

@@ -1,4 +1,8 @@
 from setuptools import setup
+import os
 
-# Metadata and options defined in setup.cfg
-setup()
+if os.getenv("READTHEDOCS", False):
+    setup(python_requires=">=3.7")
+else:
+    # Metadata and options defined in setup.cfg
+    setup()