Fix error message for the json schemas of non str-keyed mappings.

[pchanial]

The following results in an unhandled error:

from apischema.json_schema import serialization_schema
serialization_schema(dict)

File "/home/chanial/.cache/pypoetry/virtualenvs/sonouno-server-lh_mGmR0-py3.10/lib/python3.10/site-packages/apischema/json_schema/schema.py", line 221, in mapping
    if key["type"] != JsonType.STRING:
KeyError: 'type'

I have renamed the test file to be more inclusive. And xfailed one test, for which an Unsupported error is raised.

[codecov-commenter]

Codecov Report

Merging #412 (3f58a30) into master (3c60d5f) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #412      +/-   ##
==========================================
+ Coverage   87.81%   87.84%   +0.03%     
==========================================
  Files          62       62              
  Lines        5685     5685              
  Branches     1236     1236              
==========================================
+ Hits         4992     4994       +2     
+ Misses        481      480       -1     
+ Partials      212      211       -1     

Impacted Files	Coverage Δ
apischema/json_schema/schema.py	86.08% <100.00%> (+0.57%)	⬆️

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[wyfo]

Good catch, thank you!
(sorry for the merge delay, I'd taken a long break)