Do not store cached hash value when pickling

[matthiasdiener]

Fixes #284.

[matthiasdiener]

This is ready for a first look @corenting. If you like, I could add a test for this (similar to https://github.com/matthiasdiener/orderedsets/blob/main/test/test_pickle.py)

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (c9504d0) 100.00% compared to head (f3ca52d) 98.63%.

Files	Patch %	Lines
immutabledict/__init__.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #287      +/-   ##
===========================================
- Coverage   100.00%   98.63%   -1.37%     
===========================================
  Files            1        1              
  Lines           71       73       +2     
===========================================
+ Hits            71       72       +1     
- Misses           0        1       +1     

Flag	Coverage Δ
3.10	98.63% <50.00%> (-1.37%)	⬇️
3.11	98.63% <50.00%> (-1.37%)	⬇️
3.12	98.63% <50.00%> (-1.37%)	⬇️
3.8	98.63% <50.00%> (-1.37%)	⬇️
3.9	98.63% <50.00%> (-1.37%)	⬇️

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[corenting]

Thanks for the MR and for spotting the issue.
I will take a look asap.

[matthiasdiener]

Thanks @corenting ! Here is a test that could be added to this PR if you like: 2c4ac85. It fails with master, but passes with this PR.