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[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/scrapinghub/spidermon/pull/399?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub) | Coverage Δ | | |---|---|---| | [spidermon/contrib/validation/\_\_init\_\_.py](https://app.codecov.io/gh/scrapinghub/spidermon/pull/399?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub#diff-c3BpZGVybW9uL2NvbnRyaWIvdmFsaWRhdGlvbi9fX2luaXRfXy5weQ==) | `100.00% <ø> (ø)` | | | [spidermon/contrib/scrapy/pipelines.py](https://app.codecov.io/gh/scrapinghub/spidermon/pull/399?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub#diff-c3BpZGVybW9uL2NvbnRyaWIvc2NyYXB5L3BpcGVsaW5lcy5weQ==) | `82.02% <100.00%> (-1.49%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/scrapinghub/spidermon/pull/399/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub)

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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?