Fix python doc build issue (#16630)

* pin the pip versions

* remove nbconvert comment

Author: ChaiBapchya

docs/python_docs/environment.yml

@@ -27,13 +27,12 @@ dependencies:
 - matplotlib
 - notebook
 - pip:
-  # using nbconvert master until v5.5 comes out
-  - git+https://github.com/jupyter/nbconvert@master
-  - nbsphinx>=0.4.2
-  - recommonmark
-  - notedown
-  - pypandoc
-  - breathe
-  - mock
-  - awscli
-  - autodocsumm
+  - nbconvert==5.6.1
+  - nbsphinx==0.4.3
+  - recommonmark==0.6.0
+  - notedown==1.5.1
+  - pypandoc==1.4
+  - breathe==4.13.1
+  - mock==3.0.5
+  - awscli==1.16.266
+  - autodocsumm==0.1.11