Python 3.8 support (#3365)

Ensures that packages are python 3.8 compatible. 
Adds CI tests for 3.8. 
This was a work based on @seunomonije's investigations.

Fixes #3194.
Fixes #3318.

.github/workflows/ci.yml

@@ -92,7 +92,7 @@ jobs:
     name: Pytest Ubuntu
     strategy:
       matrix:
-        python-version: [ '3.6', '3.7' ]
+        python-version: ['3.6', '3.7', '3.8']
     runs-on: ubuntu-16.04
     steps:
       - uses: actions/checkout@v2
@@ -163,7 +163,7 @@ jobs:
     name: Pytest Windows
     strategy:
       matrix:
-        python-version: [ '3.6', '3.7' ]
+        python-version: [ '3.6', '3.7', '3.8' ]
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v2
@@ -182,7 +182,7 @@ jobs:
     name: Pytest MacOS
     strategy:
       matrix:
-        python-version: [ '3.6', '3.7' ]
+        python-version: [ '3.6', '3.7', '3.8' ]
     runs-on: macos-10.15
     steps:
       - uses: actions/checkout@v2

dev_tools/conf/pip-list-dev-tools.txt

@@ -8,7 +8,9 @@ pytest-benchmark~=3.2.0
 yapf~=0.27.0
 
 # For generating protobufs
-grpcio-tools~=1.24.0
+
+# 1.26 is the min version that has python 3.8 wheels
+grpcio-tools~=1.26
 mypy-protobuf==1.10
 
 # For uploading packages to pypi.
@@ -25,10 +27,6 @@ pypandoc
 myst-parser
 Sphinx~=3.2.0
 
-# have to pin otherwise RTD fails
-# see https://github.com/readthedocs/readthedocs.org/issues/7492
-Pygments==2.6.1
-
 sphinx_rtd_theme
 sphinx-markdown-tables
 
@@ -37,8 +35,5 @@ nbsphinx
 ipython
 ipykernel
 
-# For google notebooks
-google-colab
-
 # For verifying rst
 rstcheck~=3.3.1