Adding python 3.9 support

[yunhaoling]

python 3.9 is about to be released soon, we need to add python 3.9 support:

• wheels for 3.9
• enabling test for 3.9

current approach: adding python3.9 into different yaml matrix check folder here: https://github.com/Azure/azure-uamqp-python/tree/master/.azure-pipelines

[EpicWink]

Note you'll need to be wary of the tp_print slot for _typeobject structs causing issues: I'm not able to build uAMQP v1.2.11 in the official Python 3.9 Docker image

Minimal Dockerfiles (click to expand)

```Dockerfile FROM python:3.9-alpine RUN apk add cmake make g++ openssl-dev musl-dev libc-dev libstdc++ binutils RUN pip3 install 'uamqp==1.2.11' ``` ```Dockerfile FROM python:3.9-slim RUN apt-get update && apt-get install -y cmake make g++ libssl-dev RUN pip3 install 'uamqp==1.2.11' ```

[yunhaoling]

hey @EpicWink , the compiler error due to tp_print has been fixed by the latest stable cython (0.29.21), we have bumped the dependency version accordingly.

we have released uamqp 1.2.12 with support for python3.9 -- https://pypi.org/project/uamqp/1.2.12/, closing the issue now.