Fix install_requires list syntax

[maddieford]

Description

This previous change updated the install_requires list in setup.py to use 'package;platform_requirement' syntax: #3070
As pointed out by this SO post, this syntax is only supported by setuptools v36.2+: https://stackoverflow.com/questions/21082091/install-requires-based-on-python-version
As a result, when users run setup.py install with setuptools < 36.2, they are seeing the following error:
error in WALinuxAgent setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers

This PR reverts the change to use the previous syntax (check python version using sys.version_info and append packages to install_requires accordingly).

This change has been tested manually on CentOS 7.9. Automation will be added to test these changes in a future PR.

Issue #

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