Piper/update request response normalization

### What is the problem / feature ?

- While the validation for requests and responses was decoupled, the actual `Request` and `Response` objects used internally were not.
- Also, only supported request/response objects from the `requests` library when `urllib` and `urllib2` are still common.

### How did it get fixed / implemented ?

- Changed `flex.http.normalize_request` and `flex.http.normalize_response` to not have any hard ties to each other.
- Added support for `urllib` and `urllib2` responses, and the `urllib2.Request` request object.
- [ ] documentation around the supported request/response objects updated.

### How can someone test / see it ?

Code

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