Remove strenum optional dependancy

[Eixalde]

Replaced strenum by generic, Python-included Enum support. Works just like before

[codecov-commenter]

Codecov Report

Merging #604 (5f044b3) into master (e578255) will decrease coverage by 1.55%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master     #604      +/-   ##
==========================================
- Coverage   57.91%   56.36%   -1.55%     
==========================================
  Files          66       66              
  Lines       13233    13232       -1     
==========================================
- Hits         7663     7457     -206     
- Misses       5570     5775     +205     

Impacted Files	Coverage Δ
vip_hci/objects/pppca.py	69.83% <ø> (ø)
vip_hci/invprob/andromeda.py	10.70% <100.00%> (ø)
vip_hci/invprob/fmmf.py	12.16% <100.00%> (ø)
vip_hci/psfsub/framediff.py	30.23% <100.00%> (ø)
vip_hci/psfsub/llsg.py	20.09% <100.00%> (ø)
vip_hci/psfsub/loci.py	76.70% <100.00%> (ø)
vip_hci/psfsub/medsub.py	77.62% <100.00%> (ø)
vip_hci/psfsub/nmf_fullfr.py	52.15% <100.00%> (ø)
vip_hci/psfsub/nmf_local.py	80.00% <100.00%> (ø)
vip_hci/psfsub/pca_fullfr.py	51.45% <100.00%> (ø)
... and 2 more

... and 7 files with indirect coverage changes

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[VChristiaens]

Thanks @Eixalde !