Use $(PYTHON) for 'fixdialectc'

Use $(PYTHON) instead of the current Python for running 'fixdialectc'.
This makes it possible to install the 'fixdialectc' dependencies into
a Python virtual environment.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Add 'requirements.txt' for Python packages

Make it easy to install Python packages with Pip:

    pip install -r requirements.txt

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Merge pull request #15 from jvirtanen/features/virtualenv

Add support for Python virtual environments