Update to more recent version of HTTPX

[IanMeyers]

Is your feature request related to a problem? Please describe.
Current dependencies incompatible with latest versions of pytest-httpx

Describe the solution you'd like

Currently adding openapi-python-generator to project with poetry, and trying to mock responses with pytest-httpx results in this dependency issue:

    Because no versions of openapi-python-generator match >1.2.0,<1.2.1.dev1736753596 || >1.2.1.dev1736753596,<2.0.0
 and openapi-python-generator (1.2.1.dev1736753596) depends on httpx[all] (>=0.23.0,<0.24.0), openapi-python-generator (>1.2.0,<2.0.0) requires httpx[all] (>=0.23.0,<0.24.0).
(1) So, because openapi-python-generator (1.2.0) depends on httpx[all] (>=0.23.0,<0.24.0), openapi-python-generator (>=1.2.0,<2.0.0) requires httpx[all] (>=0.23.0,<0.24.0).

    Because no versions of httpx match >0.23.0,<0.23.1 || >0.23.1,<0.23.2 || >0.23.2,<0.23.3 || >0.23.3,<0.24.0
 and httpx[all] (0.23.0) depends on httpx (0.23.0), httpx[all] (>=0.23.0,<0.23.1 || >0.23.1,<0.23.2 || >0.23.2,<0.23.3 || >0.23.3,<0.24.0) requires httpx (0.23.0).
    And because httpx[all] (0.23.1) depends on httpx (0.23.1), httpx[all] (>=0.23.0,<0.23.2 || >0.23.2,<0.23.3 || >0.23.3,<0.24.0) requires httpx (0.23.0 || 0.23.1).
    And because httpx[all] (0.23.2) depends on httpx (0.23.2)
 and httpx[all] (0.23.3) depends on httpx (0.23.3), httpx[all] (>=0.23.0,<0.24.0) requires httpx (0.23.0 || 0.23.1 || 0.23.2 || 0.23.3).
    And because openapi-python-generator (>=1.2.0,<2.0.0) requires httpx[all] (>=0.23.0,<0.24.0) (1), openapi-python-generator (>=1.2.0,<2.0.0) requires httpx (0.23.0 || 0.23.1 || 0.23.2 || 0.23.3)
    And because pytest-httpx (0.35.0) depends on httpx (==0.28.*), openapi-python-generator (>=1.2.0,<2.0.0) is incompatible with pytest-httpx (0.35.0).
    So, because mrl-paddock depends on both openapi-python-generator (>=1.2.0,<2.0.0) and pytest-httpx (0.35.0), version solving failed.

Describe alternatives you've considered

Can try with pytest-requests, but would prefer httpx

Additional context
Add any other context or screenshots about the feature request here.

[dougborg]

#99 updates httpx as part of adding OpenAPI 3.1 support.