fix: Move httpcore[asyncio] from global test deps to specific envs

The asyncio extra on httpcore pulls in anyio, which conflicts with
starlette's anyio<4.0.0 pin and causes pip to downgrade httpcore to
0.18.0. That old version crashes on Python 3.14 due to typing.Union
not having __module__.

Keep httpcore[http2] in requirements-testing.txt (shared by all envs)
and add httpcore[asyncio] only to linters, mypy, and common envs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: BYK

requirements-testing.txt

@@ -11,7 +11,7 @@ asttokens
 responses
 pysocks
 socksio
-httpcore[http2,asyncio]
+httpcore[http2]
 setuptools
 Brotli
 docker

tox.ini

@@ -327,14 +327,17 @@ deps =
 
     linters: -r requirements-linting.txt
     linters: werkzeug<2.3.0
+    linters: httpcore[asyncio]
 
     mypy: -r requirements-linting.txt
     mypy: werkzeug<2.3.0
+    mypy: httpcore[asyncio]
     ruff: -r requirements-linting.txt
 
     # === Common ===
     py3.8-common: hypothesis
     common: pytest-asyncio
+    common: httpcore[asyncio]
     # See https://github.com/pytest-dev/pytest/issues/9621
     # and https://github.com/pytest-dev/pytest-forked/issues/67
     # for justification of the upper bound on pytest