Update responses to 0.25.8

[pyup-bot]

This PR updates responses from 0.25.7 to 0.25.8.

Changelog

0.25.8

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* Fix bug where the content type is always recorded as either text/plain or application/json. See 770
* Allow asserts on add_callback() matches. See 727

Links

• PyPI: https://pypi.org/project/responses
• Changelog: https://data.safetycli.com/changelogs/responses/
• Repo: https://github.com/getsentry/responses

[pyup-bot]

Closing this in favor of #308