Do not invoke pip as part of build process

Currently, Python optional dependencies are put to requriements.txt which Autotool/CMake scripts invoke pip on.
The idea was to do the same as our other language builds do: install dependencies (Java, Ruby, Haskell etc).

In retrospect, each language has different convention/tooling and it's rather surprising or rude thing to do in Python IMO.
Instead, we can moved them to "extras_require" in setup.py so that one can install via "pip install thrift[all]" if they want to.