Cherry-pick from forks of brombo/galgebra

[utensil]

There're some nice work in other forks of brombo/galgebra, here's a summary:

Has some interesting changes:

• Python 3/Installation/misc fixes
  • https://github.com/godefv/galgebra
  • https://github.com/554richard/galgebra
  • https://github.com/phlummox/galgebra (superceded by setup.py)
• leo dorst book exercices
  • https://github.com/meuns/galgebra

No particular interesting changes:

• even
  • https://github.com/alexanderpokorny/galgebra
  • https://github.com/arivero/galgebra
  • https://github.com/atakan/galgebra
  • https://github.com/gabrielraya/galgebra
  • https://github.com/johanbluecreek/galgebra
  • https://github.com/mevangelista-alvarado/galgebra
  • https://github.com/NathanielBlairStahn/galgebra
  • https://github.com/prlw1/galgebra
  • https://github.com/rowhit/galgebra
  • https://github.com/skaslev/galgebra
  • https://github.com/vliviu/galgebra
  • https://github.com/vtrandal/galgebra
• diverged
  • https://github.com/arsenovic/galgebra : -8+5
  • https://github.com/ghtdak/galgebra: -39+1
• behind
  • https://github.com/lizhen-dlut/galgebra
  • https://github.com/lzfernandes/galgebra
  • https://github.com/njisrawi/galgebra
  • https://github.com/palazzol/galgebra
  • https://github.com/parsoyaarihant/galgebra
  • https://github.com/salotz/galgebra
  • https://github.com/sashmit/galgebra
  • https://github.com/satishgoda/galgebra
  • https://github.com/tormodv/galgebra

[eric-wieser]

Can we close this given #68 tracks the only remaining fork of interest?