[v1.x] build: restrict httpx to <1.0.0

Backport of #2345 to the v1.x release branch.

httpx publishes 1.0.devN pre-releases on PyPI which are a ground-up
rewrite with an incompatible API (no AsyncClient, Auth, Timeout, or
TransportError). With the previous unbounded `httpx>=0.27.1`
constraint, `pip install --pre` of any package depending on mcp
resolves httpx 1.0.dev3, which makes `import mcp` fail at
`httpx_sse._exceptions` with:

    AttributeError: module 'httpx' has no attribute 'TransportError'

Pin to `<1.0.0` until we explicitly add httpx 1.x support.

Reported-by: SyncTekLLC
Github-Issue: #2543

Author: maxisbey

pyproject.toml

@@ -23,7 +23,7 @@ classifiers = [
 ]
 dependencies = [
     "anyio>=4.5",
-    "httpx>=0.27.1",
+    "httpx>=0.27.1,<1.0.0",
     "httpx-sse>=0.4",
     "pydantic>=2.11.0,<3.0.0",
     "starlette>=0.27",