fix: Use covariant type annotation for collection sampling type

[Oliver Borchert (borchero)]

Motivation

Providing a custom sampling type does not actually work all too well right now (or, rather, not at all) as MutableMapping is invariant. Afaict, there's no way around using Mapping here which is not quite as accurate but makes no practical difference as dataframely itself never mutates the sample (i.e. it only needs to be mutable from the user's perspective who knows the concrete type).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (54cbc75) to head (211c3af).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #9   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           36        36           
  Lines         1788      1788           
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  Hits          1788      1788           

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