Fix Python linters and clean up run-bmv2-ptf-test.py, testutils, and ebpfenv

[fruffy]

• Modernize the way we lint our Python code. Like in our C++ code, imports should be relative to the project root. The advantage is that this way linters such as pylint and mypy can also pick up the imports and do not throw errors.
• Add a mypy config to pyproject.toml and add an init hook to pylint. This way both tools understand that the folder of the pyproject.toml is the root folder.
• Fold the .pylintrc file into the pyproject.toml file. Reduces the amount of files we have in the root folder.
• Fix up typing and linting complains in run-bmv2-ptf-test.py, testutils.py, and ebpfenv.py. These fixes are a direct consequence of getting the linters to work. There were lots of subtle typing mistakes which could have lead to bugs.

There is still a lot of work left to make mypy and pylint standard but I think this is a step in the right direction.

[jafingerhut]

I looked through all of the Python code changes, and they looked reasonable to me.