Add support for httpx2

[Klavionik]

Hey! I'm trying to add support for the httpx2 package, following the guidelines from the authors.

This PR allows users of the package to seamlessly migrate their application and test code from httpx, just by updating the package. Users who have no intentions to migrate will be fine as well: the added _compat.py module tries to use httpx2 first, but falls back to httpx.

This is a highly anticipated change I assume, so I'm looking forward for a review and any comments/suggestions are very welcome.

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[bagowix]

perhaps it's worth doing this as an extra?

[Colin-b]

I would rather remove both httpx and httpx2 as dependencies. Make them optional (as the suggestion above, an extra dependency for each)

[bagowix]

I would leave this choice up to the user and not talk about the need, because migration does not happen so quickly.

"Neither `httpx2` nor the legacy `httpx` package is installed.\n"
"Install `httpx2` with:\n"
"    $ pip install httpx2\n"

[bagowix]

@Colin-b Hi! Could we prioritize this, if possible, please?

[Klavionik]

@Colin-b Kind reminder. 🙏🏻

[Colin-b]

I feel like it needs more work, I envision the following in term of support:

1. only httpx2 installed -> valid
2. only httpx installed -> valid
3. both installed -> use httpx2

This would need to be documented, and ofc the test suite will need to be updated, same for the CI that will need to run in 3 different dependency setup.

I am however not asking you to do it, I can review this proposal a bit more and add those changes.

[Klavionik]

I could work on these changes if it helps.

So, the plan would be:

1. Turn httpx and httpx2 into extra dependencies. pip install pytest-httpx will then install just this package.
2. Update the docs to explain the new ways to install pytest-httpx.
3. Update the test suite and the CI config to run in three environments: only httpx, only httpx2, httpx + httpx2. Perhaps add a test case to cover the compatibility layer itself?

Is that correct?

I envision the following in term of support:
...

I believe it works exactly this way right now.

[Colin-b]

If you feel like it, be my guest!
Note that I made a mistake in my suggestion when both are installed, and I think it makes it slightly less straightforward to handle.
When both dependencies are installed, we should mock both, I did not put thoughts into the actual implementation but I would like to avoid as much code duplication as possible.