Defer httpx ImportError to instantiation time for Sphinx compatibility

The module-level ImportError prevented Sphinx autodoc from importing
async_client when httpx is not installed. Now the module imports
successfully (setting httpx=None) and raises ImportError only when
AsyncAnticaptchaClient is instantiated without httpx.

https://claude.ai/code/session_01Pimg4VAco2v4srPeZj44Zm

Author: claude

python_anticaptcha/async_client.py

@@ -9,10 +9,7 @@
 try:
     import httpx
 except ImportError:
-    raise ImportError(
-        "httpx is required for async support. "
-        "Install it with: pip install python-anticaptcha[async]"
-    )
+    httpx = None  # type: ignore[assignment]
 
 from .exceptions import AnticaptchaException
 from .tasks import BaseTask
@@ -130,6 +127,11 @@ def __init__(
         host: str = "api.anti-captcha.com",
         use_ssl: bool = True,
     ) -> None:
+        if httpx is None:
+            raise ImportError(
+                "httpx is required for async support. "
+                "Install it with: pip install python-anticaptcha[async]"
+            )
         self.client_key = client_key or os.environ.get("ANTICAPTCHA_API_KEY")
         if not self.client_key:
             raise AnticaptchaException(