Add a network pytest mark for tests that use the network (#1669)

Add a network pytest mark for tests that use the network (#1669)

* Add a network pytest mark for tests that use the network

Sometimes it's useful to have the tests that use the network
marked so they can be skipped easily when we know the network
is not available.

This is useful for example on SUSE and openSUSE's build servers.
When building the httpx packages (actually, any package in the
distribution) the network is disabled so we can assure
reproducible builds (among other benefits). With this mark, it's
easier to skip tests that can not succeed.

* Add a better explanation for the network marker

Co-authored-by: Florimond Manca <15911462+florimondmanca@users.noreply.github.com>
Co-authored-by: Joe <nigelchiang@outlook.com>
Co-authored-by: Florimond Manca <15911462+florimondmanca@users.noreply.github.com>
Co-authored-by: Tom Christie <tom@tomchristie.com>