Responses

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* Fixed packages not containing type stubs.

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* `responses.upsert()` was added. This method will `add()` a response if one
has not already been registered for a URL, or `replace()` an existing
response.
* `responses.registered()` was added. The method allows you to get a list of
the currently registered responses. This formalizes the previously private
`responses.mock._matches` method.
* A more useful `__repr__` has been added to `Response`.
* Error messages have been improved.

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* `responses.urlencoded_params_matcher` and `responses.json_params_matcher` now
accept None to match empty requests.
* Fixed imports to work with new `urllib3` versions.
* `request.params` now allows parameters to have multiple values for the same key.
* Improved ConnectionError messages.

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- Remove support for Python 3.4.

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- Added the `match` parameter to `add()`.
- Added `responses.urlencoded_params_matcher()` and `responses.json_params_matcher()`.

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- Add a requirements pin to urllib3. This helps prevent broken install states where
cookie usage fails.