Fix slow Sage imports

[grhkm21]

By replacing all Sage imports with an individual imoprt, the loading time for from estimator import * reduces from 3.5s to 1.4s. Doctests seem to work (except for timeout stuffs) so I assume I didn't break anything 😄

[bencrts]

I guess we would need the CI to pass before we used an alternative technique for importing (see here: https://github.com/malb/lattice-estimator/pull/76/checks). It seems to be unhappy with the way infinity is imported (I can't tell why at first look).

[grhkm21]

The PEP8 fails because of F401 'sage.rings.all.QQ' imported but unused, but this is necessary due to Sage shenanigans. I just overwrote it with # noqa, which allows the check to be performed for other import statements.

Regarding the infinity import problem, I have no idea why. It works for me locally:

sage -python -c "from sage.rings.infinity import infinity as oo" 

[malb]

Perhaps the CI uses a different Sage version? Those deep imports are not supported by Sage, ie it gives no guarantee those will work.

[grhkm21]

The most recent log prints this at the very start:

Run export SAGE_ROOT=`sage -c "import os; print(os.environ['SAGE_ROOT'])" | tail -1`
Traceback (most recent call last):
  File "/usr/bin/sage-eval", line 10, in <module>
    eval(compile(s,'<cmdline>','exec'))
  File "<cmdline>", line 1, in <module>
  File "/usr/lib/python3.10/os.py", line 680, in __getitem__
    raise KeyError(key) from None
KeyError: 'SAGE_ROOT'

which seems weird. Also, CI seems to specify that Sage is installed via apt-get install, which from my past experience severely lags behind. It says on this website that it can be anywhere from Sage 9.0 to 10.0...
Is it possible to make CI use the sage docker image instead, or would that be too much work? This PR isn't that important after all

[malb]

Indeed, this is installing sagemath (9.5-4). I'm hesitant to make changes that will break on Sage versions this recent, just imagine all the panicking support requests we'd have to deal with.

[grhkm21]

This is very weird, I tried using a fresh Sage 9.0 (conda) installation and there's no issue with importing. Not sure then...

I also noticed that at the start of the log, it prints an error where SAGE_ROOT env variable is not set, not sure if that is related.

I will close this issue but if I figure out why this is happening I will report back

[malb]

FWIW, removing the SAGE_ROOT stuff from the tests doesn't affect them: #81