pin python deps version for building docs (#2154)

.travis.yml

@@ -70,7 +70,7 @@ matrix:
         # Install Python dependencies for building docs
         - which python
         - python -V
-        - pip install -U -q "sphinx>=3.0" sphinx-rtd-theme nbsphinx Pillow
+        - pip install -U -q "sphinx==3.1.2" "sphinx-rtd-theme==0.5.0" "nbsphinx==0.7.1" "Pillow==7.2.0"
         # m2r needs a patch for sphinx 3
         # https://github.com/sphinx-doc/sphinx/issues/7420
         - pip install -U -q git+https://github.com/intel-isl/m2r@dev#egg=m2r

docs/builddocs.rst

@@ -40,19 +40,13 @@ First, install a TeX distribution such as `MacTeX <http://www.tug.org/mactex/>`_
 
 .. code-block:: bash
 
-    # For pip
-    pip install "sphinx>=3.0" sphinx-autobuild nbsphinx Pillow
+    # Install deps
+    pip install -U "sphinx==3.1.2" "sphinx-rtd-theme==0.5.0" "nbsphinx==0.7.1" "Pillow==7.2.0"
 
-    # Or, for Conda
-    conda install "sphinx>=3.0" sphinx-autobuild nbsphinx Pillow
-
-Install the patched ``m2r`` package for converting ``.md`` to ``.rst``.
-
-.. code-block:: bash
-
-    # https://github.com/sphinx-doc/sphinx/issues/7420
+    # Patched m2r: https://github.com/sphinx-doc/sphinx/issues/7420
     pip install -U -q git+https://github.com/intel-isl/m2r@dev#egg=m2r
 
+
 Build
 -----