azdev setup: Pin pip to 20.2.4

[jiasli]

Due to pip bug

• pip 20.3.2 eagerly downloads all versions of a package during dependency resolution pypa/pip#9284
• python -m pip install --upgrade pip goes into backtracking since 20.3.2 pypa/pip#9285

azdev setup: Pin pip to 20.2.4, as Azure CLI repo does:

https://github.com/Azure/azure-cli/blob/c18f5ad9a1fe66ea5e47b7c672c564311f986913/azure-pipelines.yml#L364

    - bash: pip install --upgrade pip==20.2.4 tox

[jiasli]

This issue has been fixed in pip 20.3.3.