Fix _get_metadata_class to work recursively

[wRAR]

A straightforward fix for #49 (at least assuming I've implemented BFS correctly), but I don't have a clear picture on what do __orig_bases__ and __origin__ do yet. Note that __mro__ isn't suitable as it's depth-first.

[wRAR]

Requires new web-poet (scrapinghub/web-poet#184)

[codecov-commenter]

Codecov Report

Merging #51 (bbdf100) into main (a3eb9b6) will not change coverage.
The diff coverage is 0.00%.

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Additional details and impacted files

@@          Coverage Diff          @@
##            main     #51   +/-   ##
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  Coverage   0.00%   0.00%           
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  Files          8       8           
  Lines        731     729    -2     
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+ Misses       731     729    -2     

Files Changed	Coverage Δ
zyte_common_items/pages.py	0.00% <0.00%> (ø)

[kmike]

Thanks @wRAR!