Add conversion to bool for Maybe

[InAnYan]

I have made things!

1. Add __bool__ methods to Maybe (unfilled), Something and _Nothing.
2. Add a new test file for that conversion.

Checklist

• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc)
• I have created at least one test case for the changes I have made
• I have updated the documentation for the changes I have made
• I have added my changes to the CHANGELOG.md

Related issues

Closes #2177.

[InAnYan]

Hope I wrote code according to your conventions.

But I'm not sure:

1. I think it's not necessary to add this to CHANGELOG.md, as this PR is quite small.
2. I'm not sure about the docs -- what would be the best place to explain this? Or maybe skip this too

[sobolevn]

Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (82ef3ef) to head (30aa760).
Report is 360 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master     #2178    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           80        81     +1     
  Lines         2485      2581    +96     
  Branches       437        44   -393     
==========================================
+ Hits          2485      2581    +96     

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