requirements: Unpin pygments from ~=2.15.1 to >=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.

This was temporarily fixed by pinning pygments at ~=2.15.1 in zulip#1433.

Method `translate_styles` manually converts the pygments `bold italic`
style into the urwid-compatible `bold, italics` is there.

Unpin Pygments from ~=2.15.1 to >=2.14.0.

Fixes part of zulip#1434.

setup.py

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