Deprecate schematics

[mrwbarg]

Addresses parts of #370.

Deprecate the use of schematics as it's unlikely to receive support after Python 3.11.
This PR only removes schematics but doesn't replace it with another library (as we already have support for jsonschema).
A future PR can add support to another library such as Pydantic for example.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.17 ⚠️

Comparison is base (44d5316) 76.54% compared to head (ef9e765) 76.37%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
- Coverage   76.54%   76.37%   -0.17%     
==========================================
  Files          76       73       -3     
  Lines        3214     3103     -111     
  Branches      384      365      -19     
==========================================
- Hits         2460     2370      -90     
+ Misses        683      663      -20     
+ Partials       71       70       -1     

Impacted Files	Coverage Δ
spidermon/contrib/validation/__init__.py	100.00% <ø> (ø)
spidermon/contrib/scrapy/pipelines.py	82.02% <100.00%> (-1.49%)	⬇️

... and 1 file with indirect coverage changes

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[VMRuiz]

I'm in favor of deprecating this, but should we give some warnings first? We can add some warning messages saying that schematics support will be drop in futures versions.

[mrwbarg]

Agreed. It's just that schematics support is already dropped. How often are new versions released that we can do thiso gradually?