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

The previous commit fixes the issue #1431 that was temporarily
fixed by pinning pygments at ~=2.15.1 in #1433.

Subsequent versions of pygments have since been released, but an upper
limit of 2.18.0 is included, 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",