Dict: implement the __contains__ method

[sphuber]

Fixes #5326

This now allows to do the following:

'some_key' in Dict({'value'})

and is guaranteed to return a boolean, instead of raising a KeyError
if the key doesn't exist, which was the previous behavior.

[sphuber]

@louisponet

[codecov]

Codecov Report

Merging #5328 (81f9e0e) into develop (fe1acf9) will decrease coverage by 0.03%.
The diff coverage is 79.60%.

@@             Coverage Diff             @@
##           develop    #5328      +/-   ##
===========================================
- Coverage    82.13%   82.11%   -0.02%     
===========================================
  Files          533      533              
  Lines        38478    38427      -51     
===========================================
- Hits         31601    31549      -52     
- Misses        6877     6878       +1     

Flag	Coverage Δ
django	77.18% <79.60%> (-0.03%)	⬇️
sqlalchemy	76.48% <78.24%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiida/cmdline/commands/cmd_setup.py	56.87% <0.00%> (+5.99%)	⬆️
aiida/cmdline/commands/cmd_status.py	84.77% <ø> (ø)
aiida/cmdline/params/options/commands/setup.py	54.55% <0.00%> (-2.29%)	⬇️
aiida/cmdline/params/types/profile.py	64.45% <0.00%> (ø)
aiida/manage/configuration/__init__.py	83.73% <50.00%> (+0.13%)	⬆️
aiida/manage/configuration/config.py	89.48% <71.43%> (+0.30%)	⬆️
aiida/manage/external/postgres.py	63.10% <75.00%> (+0.14%)	⬆️
aiida/backends/utils.py	93.34% <83.34%> (-6.66%)	⬇️
aiida/manage/configuration/profile.py	89.60% <84.91%> (-6.67%)	⬇️
...iida/manage/configuration/migrations/migrations.py	93.89% <90.91%> (-0.35%)	⬇️
... and 10 more

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[mbercx]

All good, thanks @sphuber!

EDIT: Well, once you can appease the codecov Gods. 😅