Allow building from a Pipfile or requirements.txt

[henryiii]

Currently, you can't include pylandau in a requirements listing for a virtual environment, like pipenv, since it requires that numpy be pre-installed. This patch should lift that requirement.

[henryiii]

Fixed and ready for merging if it looks good. :)

[DavidLP]

Thank you very much. The issue is that if numpy is not installed the setup fails since it is imported too early and If one would install numpy first and then pylandau it works, no? Ok, this is an important enhancement, especially since pip install numpy does actually work nowadays.

[henryiii]

Yes, so pip install numpy pylandau fails, while pip install numpy && pip install pylandau works, but in a requirements list or similar the user doesn't always have that luxury.