Define valid HTTP method depending on openapi version

[lafrech]

See #165 (comment).

Here are two different implementations (see intermediate commit). Tell me which you prefer, or if you prefer a third approach.

Let's not overthink this too much.

Do you think we need to add a test?

[lafrech]

On second thought, the first approach

VALID_METHODS = {
    2: VALID_METHODS_OPENAPI_V2,
    3: VALID_METHODS_OPENAPI_V3,
}

is more in line with the rest of clean_operation

        ref_paths = {
            'parameter': {
                2: 'parameters',
                3: 'components/parameters',
            },
            'response': {
                2: 'responses',
                3: 'components/responses',
            },
        }

I can force-reset the second commit.

[sloria]

Yeah, let's go with the first approach. Go ahead and merge after you've done the reset.