Change `linalg.dsolve.factorized` to `linalg.factorized`

[A-CGray]

Purpose

SciPy has started producing the following deprecation warnings when running pySpline:

DeprecationWarning: Please use `factorized` from the `scipy.sparse.linalg` namespace, the `scipy.sparse.linalg.eigen` namespace is deprecated.

This PR updates where we get the factorized function from to fix this. I checked, and the factorized function is available in the new location in scipy 1.5.4 which is the oldest version we currently we support.

Expected time until merged

1 day

Type of change

• [x] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (non-backwards-compatible fix or feature)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no API changes)
• [ ] Documentation update
• [ ] Maintenance update
• [ ] Other (please describe)

Testing

Checklist

• [x] I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• [ ] I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• [x] I have run unit and regression tests which pass locally with my changes
• [ ] I have added new tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation

[codecov[bot]]

Codecov Report

Merging #53 (2304674) into main (0c78591) will not change coverage. The diff coverage is 42.85%.

@@           Coverage Diff           @@
##             main      #53   +/-   ##
=======================================
  Coverage   52.53%   52.53%           
=======================================
  Files           5        5           
  Lines        1616     1616           
=======================================
  Hits          849      849           
  Misses        767      767           

Impacted Files	Coverage Δ
pyspline/pyVolume.py	40.80% <0.00%> (ø)
pyspline/pySurface.py	63.24% <50.00%> (ø)
pyspline/pyCurve.py	51.87% <66.66%> (ø)

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