Fix doc generation with Python 3.6

Author: vkarak

docs/requirements.txt

@@ -1,6 +1,7 @@
 archspec==0.2.1
 docutils==0.17.1 # https://github.com/sphinx-doc/sphinx/issues/9001
 jsonschema==3.2.0
-semver==3.0.0
+semver==2.13.0; python_version == '3.6'
+semver==3.0.0; python_version >= '3.7'
 Sphinx==5.3.0
 sphinx-rtd-theme==1.2.1