**- Why I did it**
To install the framework for adding unit tests to the sonic-py-common package and report coverage.
** How I did it **
- Incorporate pytest and pytest-cov into sonic-py-common package build
- Updgrade version of 'mock' installed to version 3.0.5, the last version which supports Python 2. This fixes a bug where the file object returned from `mock_open()` was not iterable (see https://bugs.python.org/issue32933)
- Add support for Python 3 setuptools and pytest in sonic-slave-buster environment
- Add tests for `device_info.get_machine_info()` and `device_info.get_platform()` functions
- Also add a .gitignore in the root of the sonic-py-common directory, move all related ignores from main .gitignore file, and add ignores for files and dirs generated by pytest-cov
