requirements: Unpin pygments from ~=2.15.1 to >=2.14.0,<2.18.0.

The previous commit in #1452 fixes the issue #1431 that was temporarily
fixed by pinning Pygments at ~=2.15.1.

Combined with <2.18.0, since that unreleased version depends on
Python3.8 and could make ZT uninstallable on older pythons.

Fixes part of #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,<2.18.0",
         "typing_extensions~=4.5.0",
         "python-dateutil>=2.8.2",
         "pytz>=2022.7.1",