test tensorflow2.2rc3 on python3.8 (#3839)

* use py3.8, install cmake for onnx

* disable other circleci tests for now

* install proto for onnx install

* apt-get install protobuf-compiler instead

* skip onnx for python3.8

* use right comparison

* fix up config

* changelog and faq

.circleci/config.yml

           pyversion: 3.7.3
           # Test python 3.7 with the newest supported versions
           pip_constraints: test_constraints_max_tf2_version.txt
+      - build_python:
+          name: python_3.8.2+tf2.2
+          executor: python382
+          pyversion: 3.8.2
+          # Test python 3.8 with the newest edge versions
+          pip_constraints: test_constraints_max_tf2_version.txt
       - markdown_link_check
       - pre-commit
       - deploy:

com.unity.ml-agents/CHANGELOG.md

 - `num_updates` and `train_interval` for SAC were replaced with `steps_per_update`. (#3690)
 - `WriteAdapter` was renamed to `ObservationWriter`. If you have a custom `ISensor` implementation,
 you will need to change the signature of its `Write()` method. (#3834)
+- The maximum compatible version of tensorflow was changed to allow tensorflow 2.1 and 2.2. This
+will allow use with python 3.8 using tensorflow 2.2.0rc3.
 - `UnityRLCapabilities` was added to help inform users when RL features are mismatched between C# and Python packages. (#3831)
 
 ### Bug Fixes

docs/FAQ.md

 - You're using 32-bit python instead of 64-bit. See the answer
   [here](https://stackoverflow.com/a/1405971/224264) for how to tell which you
   have installed.
-- You're using python 3.8. Tensorflow plans to release packages for this as soon
-  as possible; see
-  [this issue](https://github.com/tensorflow/tensorflow/issues/33374) for more
-  details.
 - You have the `tensorflow-gpu` package installed. This is equivalent to
   `tensorflow`, however `pip` doesn't recognize this. The best way to resolve
   this is to update to `tensorflow==1.15.0` which provides GPU support in the

test_constraints_max_tf2_version.txt

 # For projects with upper bounds, we should periodically update this list to the latest release version
 grpcio>=1.23.0
 numpy>=1.17.2
-tensorflow>=2.1.0
+tensorflow==2.2.0rc3
 h5py>=2.10.0

test_requirements.txt

 pytest-cov==2.6.1
 pytest-xdist
 
-tf2onnx>=1.5.5
+# onnx doesn't currently have a wheel for 3.8
+tf2onnx>=1.5.5;python_version<'3.8'