requirements: Upgrade urwid_readline from >=0.13 to >=0.15.1.

Author: neiljp

setup.py

@@ -107,7 +107,7 @@ def long_description():
     install_requires=[
         "urwid~=2.1.2",
         "zulip>=0.8.2,<0.9.0",  # Next release, 0.9.0, requires Python 3.9
-        "urwid_readline>=0.13",
+        "urwid_readline>=0.15.1",
         "beautifulsoup4>=4.11.1",
         "lxml~=4.9.2",
         "pygments>=2.14.0,<2.18.0",