Fix Biopython installation with pip

Hi all;
This is yet another take on making Biopython install nicely with pip in virtual environments. This avoids adding numpy as an explicit dependency and instead uses it if present or skips it if not.

The problem with the previous install_requires approach is that pip doesn't build and install all requirements before setting up Biopython, so Biopython will fail with a numpy missing error. Additionally, our old approach drags in numpy so creates a heavyweight dependency for isolated environments.

The new approach requires users to explicitly install numpy if needed but doesn't penalize them if it's not present.

I submitted as a pull request for documentation and feedback from anyone. If y'all agree, merge away. Thanks,
Brad