Requests via `httpx`

[JR-1991]

Overview

As highlighted and discussed in issue #171, the requests library seems to supply wrong content types, leading to certain upload operations such as replace_datafile not working (see the issue for details). This PR addresses this issue and substitutes requests with httpx that fixes the highlighted problem.

Besides the previously mentioned fix, the library also provides means of async requests as shown here. Since async requests were planned anyway, this would inevitably also lead to ditching requests and thus the change to httpx already paves the way.

Changes introduced by this PR

• Substituted base requests get_request, post_request, put_request and delete_request with httpx requests
• Update requirements to include httpx>=0.26.0
• Added tests for datafile upload and replacement
  • test_file_replacement addresses an example given in issue #171

Additional notes

• Due to httpx supporting Python >3.8, tests for 3.7 had to be removed

Closes

Fixes #171

[JR-1991]

Todo: Add a badge for Python versions supported