Make Pythran a required build dependency

[lucascolley]

@rgommers

did have a quick search for use of -Duse-pythran=false, and found that two distros build with that flag set by default:

• Termux: https://github.com/termux/termux-packages/blob/ and https://github.com/knyipab/tur/blob/ce123f82206744f77dfb7166c5d2a21c25dacefd/tur/python-scipy/build.sh#L31fa856888326f1087c31feaaf15164984f872d3fb/packages/python-scipy/build.sh#L22
• PTXdist: ptxdist/rules/python3-scipy.make at 65365688811e7081a6b604be1f41c5ccdaf7e75a · LeSpocky/ptxdist · GitHub

Before taking this decision, it should be investigated what the problem exactly is there.

Did anyone reach out to Termux/PTXdist? It looks like the authors of those flags are licy183 (Chongyun Lee) · GitHub and michaelolbrich (Michael Olbrich) · GitHub respectively.