Investigate pip installation in CI

[yunhaoling]

We have a step in our client/test.yml called Install dependencies where we will download the get-pip.py scripts and install the pip.

however, with the release of pip 21, it only supports python >= 3.6, which will lead to pip installation failure on python 2.7 and 3.5 with the latest get-pip.py. The current workaround we apply is adding a new [GetPip][https://github.com/Azure/azure-uamqp-python/blob/633ae897af35f3b003edae47c960fa90e5add1ef/.azure-pipelines/client.yml#L11) variable in the yml, and override the value in python27 and 35 matrix.

However, we might not want to install the pip through the get-pip.py script at the beginning for the sake of scalability. What we could try is

• replace the fetching get-pip.py with python -m pip install --upgrade pip
• or we pin the pip to a specific version like 20.2

cc: @lmazuel

[yunhaoling]

it's not an issue anymore as we dropped off python2.7 and 3.5