feat: add delay before response

[lucsorel]

hello @amaziahub, thank you for the mimicker project 🙏

Here is a pull request to add the route.delay(duration) optional feature to make a matched stub delay before returning the response. This feature allows to test how the code (calling a mimicked web API) reacts to a connection timeout.

I added:

• some documentation in the README.md file
• some unit tests about the route definition and to check that the mimicker server actually waits when asked to

This feature and the tests are included in the same commit. Other commits are suggestions that can be reverted if you like:

• I moved requests in the dev dependencies because it is not used in the production code: it thus won't be installed in the codebases using mimicker (requests can conflict with other web clients like niquests or httpx)
• I made mimicker able to run several times in a row so that tests can be run multiple times
• I created a namedtuple class to model a Stub to increase code legibility and add type annotations in the code

Also, the first commit was necessary to fix some syntax errors in an f-string.

I tried to follow the code standards of the project and I used hamcrest in the unit tests (thanks for the discovery), I hope you will appreciate this contribution. Thank you again for mimicker 🙂