Install step broken for Python 3

[Prof Garth Wells]

The file config/install.py is not Python 3 compatible (command module was removed in Python 3), which leads to install failure if the interpreter python is Python 3.

The proper fix would be to make the SLEPc build system Python 3 compatible. (Python 2 reaches end-of-life in under 12 months).

A hack would be to ensure that config/install.py uses the same interpreter as the configure step. The configure step presently asks the user to run

python2 './configure' '--prefix=foo . . . .

in the case that python is Python 3.

[Jose E. Roman]

I have pushed a fix in branch jose/maint/python3-install. Probably not the best solution but it works. If it is ok, I will merge to maint and master.

My plan is to migrate all SLEPc scripts to python3, but this will have to wait for about 2 or 3 weeks.

[Jose E. Roman]

Merged.

I will close this issue and fix the rest of SLEPc scripts at a later time.