[deps-] make zoneinfo an optional dependency

Add missing backport dep #2094

Thanks @pacien!

requirements.txt

@@ -39,6 +39,7 @@ psutil              # procmgr
 numpy               # npy pandas hdf5 arrow
 tomli; python_version < '3.11' # toml
 pyconll             # conll/conllu
+backports.zoneinfo; python_version < '3.9' #f5log
 
 requests_cache      # scraper
 beautifulsoup4      # scraper

visidata/loaders/f5log.py

@@ -4,10 +4,6 @@
 from datetime import datetime, timedelta
 from ipaddress import ip_address
 
-try:
-    import zoneinfo
-except ImportError:
-    from backports import zoneinfo
 import re
 import traceback
 from typing import Any, Dict, Optional
@@ -1043,6 +1039,10 @@ def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         # the default F5 logs don't have the year so we have to guess from the file ctime
         # TODO: make this overridable
+        try:
+            import zoneinfo
+        except ImportError:
+            from backports import zoneinfo
         self._log_tz = zoneinfo.ZoneInfo("UTC")
         try:
             self._year = int(