[3.8] Doc: Use walrus operator in example. (GH-15934) (GH-15936)

(cherry picked from commit e1d455f3a3b82c2e08d5e133bcbab5a181b66cfb)

Co-authored-by: Julien Palard <julien@palard.fr>
diff --git a/Doc/library/http.client.rst b/Doc/library/http.client.rst
index 48bc35c..db26d56 100644
--- a/Doc/library/http.client.rst
+++ b/Doc/library/http.client.rst
@@ -516,10 +516,7 @@
    >>> # The following example demonstrates reading data in chunks.
    >>> conn.request("GET", "/")
    >>> r1 = conn.getresponse()
-   >>> while True:
-   ...     chunk = r1.read(200)  # 200 bytes
-   ...     if not chunk:
-   ...          break
+   >>> while chunk := r1.read(200):
    ...     print(repr(chunk))
    b'<!doctype html>\n<!--[if"...
    ...