Have coefficient compilation use relative path

[Ericgig]

Description
A change in setuptools (pypa/setuptools#3521) raise a warning when compiling coefficient using absolute path, which breaks our tests for dev.major:

DeprecationWarning: Absolute path '/home/runner/.qutip/qutip_coeffs_1.1/qtcoeff_8208be2129365a87dda9c12028c457' is being replaced with a relative path 'home/runner/.qutip/qutip_coeffs_1.1/qtcoeff_8208be2129365a87dda9c12028c457' for outputs. This behavior is deprecated. If this behavior is desired, please comment in pypa/distutils#169.

This changes the path to be relative.

I change the path before creating the lock files, which otherwise were polluting the active directory.

[hodgestar]

Thanks for finding and fixing this.

[hodgestar]

I assume we need a similar fix for master?

[Ericgig]

I don't think we need a fix for master, I haven't seen the test fail yet.
In master, we create the file in the active directory and use pyx import. So the absolute path should never be used.

[hodgestar]

The last build on master failed with:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/runner/work/qutip/qutip/qutip/__init__.py", line 64, in <module>
    import qutip.cy.pyxbuilder as _pyxbuilder
  File "/home/runner/work/qutip/qutip/qutip/cy/__init__.py", line 1, in <module>
    from qutip.cy.spmatfuncs import *
ModuleNotFoundError: No module named 'qutip.cy.spmatfuncs'

While trying to run qutip.about(). See https://github.com/qutip/qutip/actions/runs/2843095517. I triggered rerunning the Python 3.9 default build to see it if was temporary.

[hodgestar]

It looks like the Python 3.9 build managed to start running the tests, so hopefully we are good!