Switch from frozendict to immutabledict

[alexhenrie]

frozendict is broken on the latest versions of Python and the project is dead, see slezica/python-frozendict#30

[codecov]

Codecov Report

Merging #90 into master will increase coverage by 0.52%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
+ Coverage   80.38%   80.91%   +0.52%     
==========================================
  Files          28       28              
  Lines        2565     2635      +70     
  Branches      471      471              
==========================================
+ Hits         2062     2132      +70     
  Misses        379      379              
  Partials      124      124              

Impacted Files	Coverage Δ
pronto/relationship.py	76.32% <50.00%> (ø)
pronto/term.py	82.66% <100.00%> (ø)
pronto/parsers/rdfxml.py	69.08% <0.00%> (+0.07%)	⬆️
pronto/utils/meta.py	71.42% <0.00%> (+0.31%)	⬆️
pronto/parsers/obojson.py	88.00% <0.00%> (+0.50%)	⬆️
pronto/parsers/obo.py	86.36% <0.00%> (+0.64%)	⬆️
pronto/parsers/_fastobo.py	92.33% <0.00%> (+2.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c370293...fc6b2ef. Read the comment docs.

[althonos]

Thanks for the heads-up, didn't know frozendict was not maintained anymore!

Ultimately we'll probably have to implement a custom collections.abc.Mapping to store the relationships for a Term or a Relationship as discussed briefly in #89 .