Add virtualenv support for Python linters (#38)

PythonExternalLinter is a new abstract base class for external
Python-based linters. It introduces support for virtualenv-based tool
installations. If a known virtualenv is found in the project root, and
the linter binary can be found within it, then that path will be used to
execute the linter. Otherwise, the linter binary is assumed to be in the
$PATH.

The list of known virtualenvs defaults to .venv but can be changed
using the python.virtualenvs configuration key.