requirements: Pin pygments at ~=2.15.1 instead of >=2.14.0.

Pygments 2.16.0 introduced a style to support a combination of bold and
italic styling in pygments/pygments#2444. Both of our gruvbox themes and
the light native theme gain a 'bold strong' style via pygments as a
result, which urwid fails to parse and blocks the application from
loading.

Longer-term we should improve the pygments to urwid translation logic to
allow these styles to work and an upgrade to later pygments versions,
but for now this allows these themes to continue working as before.

Fixes zulip#1431.

setup.py

@@ -100,7 +100,7 @@ def long_description():
         "urwid_readline>=0.13",
         "beautifulsoup4>=4.11.1",
         "lxml>=4.9.2",
-        "pygments>=2.14.0",
+        "pygments~=2.15.1",
         "typing_extensions~=4.5.0",
         "python-dateutil>=2.8.2",
         "pytz>=2022.7.1",