Add options for handling [extras] dependencies

[devmotion]

This PR extends #502 based on @DilumAluthge's suggestions in #502 (comment).

I added an option open_prs_for_extras that can be set to IfExistingCompatExtras() (the default), AllExtras() or NoExtras() [my gut feeling is that the names could be improved a bit]. With IfExistingCompatExtras(), only [extras] dependencies with existing compat entry are updated; with AllExtras(), all [extras] dependencies are updated (also those without existing compat entry); and with NoExtras(), [extras] dependencies are not updated.

I went with singleton types (basically a type-based enum) because 1) it felt more consistent with the existing EntryType and 2) it's not necessary to check whether the provided value is valid (which would be necessary if the API would be based on Symbols).

Fixes #166. Closes #502.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7ea4330) to head (98a2684).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #528   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           12        11    -1     
  Lines          412       417    +5     
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+ Hits           412       417    +5     

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[DilumAluthge]

Could we make this experimental for now? That way, we can try it out, and if we don't like the API, we can change it?

[DilumAluthge]

By "experimental", I mean explicitly document that we are allowed to break it in a minor (or even patch) release of CompatHelper.

[devmotion]

I added a note.

[DilumAluthge]

Any particular reason for moving Baz from [deps] to [extras]?

[devmotion]

I was just not creative enough and hence initially only moved around things. Only later I noticed that an additional dependency is needed to cover all cases.

[devmotion]

@DilumAluthge this PR is ready for another round of review 🙂