feat(parser): Detect OpenAPI documents of incorrect versions (closes #281)

[dbanty]

1. Adds semver validation to the openapi field and removes the default (it's a required field)
2. Separate and re-export the vendored code so it retains its own license but our modifications can fall under our license
3. Enforce that the major semver component of the openapi field is 3
4. If schema validation fails, check for the swagger attribute at the top level and provide a hint to the user that they might be using an older, unsupported version.

[codecov]

Codecov Report

Merging #302 (0a0cb88) into main (aede9be) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #302   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           46        47    +1     
  Lines         1348      1380   +32     
=========================================
+ Hits          1348      1380   +32     

Impacted Files	Coverage Δ
...lient/schema/openapi_schema_pydantic/components.py	100.00% <ø> (ø)
...n_client/schema/openapi_schema_pydantic/contact.py	100.00% <ø> (ø)
...nt/schema/openapi_schema_pydantic/discriminator.py	100.00% <ø> (ø)
..._client/schema/openapi_schema_pydantic/encoding.py	100.00% <ø> (ø)
...n_client/schema/openapi_schema_pydantic/example.py	100.00% <ø> (ø)
.../openapi_schema_pydantic/external_documentation.py	100.00% <ø> (ø)
...on_client/schema/openapi_schema_pydantic/header.py	100.00% <ø> (ø)
...thon_client/schema/openapi_schema_pydantic/info.py	100.00% <ø> (ø)
...n_client/schema/openapi_schema_pydantic/license.py	100.00% <ø> (ø)
...thon_client/schema/openapi_schema_pydantic/link.py	100.00% <ø> (ø)
... and 23 more

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[emann]

One question but other that that LGTM

[emann]

Why not just check for this or the openapi field before attempting to parse the dict?

[dbanty]

1. I don't think it's illegal to have extra fields (e.g. swagger) so I don't want to disallow that if the spec is otherwise valid
2. Validating the format of openapi seems like a task for Pydantic since the semver requirement is part of the OpenAPI spec.