#### Why I did it
To ensure any environment variables which are configured in the build/test environment do not influence the behavior of sonic-py-common during unit tests. For example, variables which might be set by continuous integration pipelines.
#### How I did it
Add class-scoped pytest fixture to `TestDeviceInfo` class which stashes the current environment variables, clears them and yields. Once all the test cases in the class finish, the fixture will restore the original environment variables.
Also remove unnecessary unittest-style setup and teardown functions from interface_test.py
**- 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
