add support for providing a custom `requests.Session` in client

[syphar]

Checklist for your PR

• [ ] please start with a draft PR and mark it as ready for review when you are confident that your implementation works as expected.
• [ ] add a changelog entry in CHANGELOG.md with a link to this PR.
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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (b5ead78) 87.31% compared to head (51915ea) 87.32%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #39 +/- ## ========================================== + Coverage 87.31% 87.32% +0.01% ========================================== Files 7 7 Lines 946 947 +1 ========================================== + Hits 826 827 +1 Misses 120 120 ```

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[syphar]

@klamann we found that this feature would have helped us writing cleaner code.

In our case we wanted retries for HTTP requests, and with the option to hand in a custom session this would have worked more easily.

Another usecase I can imagine for this would be proxies.

If the general feature is fine with you, I'll do tests & changelog.

[klamann]

I like the idea, and the change seems to be simple enough. I'm willing to merge this if you add a test case and changelog entry.

[syphar]

@klamann I updated the changelog, and added a short test with a custom session.

[klamann]

sorry for the deplay; merging!