PythonParser: should parse .py files

[nokome]

The PythonParser class scan for parse Python files in a folder and extract a list of unique (non-standard-library) package dependencies from import statements.

See browserify/detective#8 as an example of using JS regexes to parse for requires. A similar approach could be used for Python import statements (since we wont have a python AST to walk in TypeScript) - there are probably already regexes out there for parsing Python import statements