requirements: Limit zulip upgrades from >=0.8.2 until Python 3.9.

Zulip 0.9.0 library release requires Python 3.9.

Author: neiljp

setup.py

@@ -106,7 +106,7 @@ def long_description():
     tests_require=testing_deps,
     install_requires=[
         "urwid~=2.1.2",
-        "zulip>=0.8.2",
+        "zulip>=0.8.2,<0.9.0",  # Next release, 0.9.0, requires Python 3.9
         "urwid_readline>=0.13",
         "beautifulsoup4>=4.11.1",
         "lxml~=4.9.2",