Revert "ref(tox): remove anyio version pin"

This reverts commit b7e32a8.

Author: srothh

tox.ini

@@ -376,6 +376,7 @@ deps =
     httpx-v0.25: pytest-httpx==0.25.0
     httpx: pytest-httpx
     # anyio is a dep of httpx
+    httpx: anyio<4.0.0
     httpx-v0.16: httpx~=0.16.0
     httpx-v0.18: httpx~=0.18.0
     httpx-v0.20: httpx~=0.20.0