fix UTCTimeField timezone

[douniwan5788]

Checklist:

• If you are new to Scapy: I have checked CONTRIBUTING.md (esp. section submitting-pull-requests)
• I squashed commits belonging together
• I added unit tests or explained why they are not relevant
• I executed the regression tests (using cd test && ./run_tests or tox)
• If the PR is still not finished, please create a Draft Pull Request

fixes #4557

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.71%. Comparing base (6f0faf3) to head (bf07e36).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4564      +/-   ##
==========================================
- Coverage   81.61%   78.71%   -2.91%     
==========================================
  Files         356      331      -25     
  Lines       85545    80350    -5195     
==========================================
- Hits        69819    63245    -6574     
- Misses      15726    17105    +1379     

Files with missing lines	Coverage Δ
scapy/data.py	88.30% <100.00%> (-1.98%)	⬇️
scapy/fields.py	92.76% <100.00%> (-0.01%)	⬇️

... and 282 files with indirect coverage changes

[gpotter2]

What protocols would use this? It seems weird to me to specify a time zone in the protocol field... especially in the UTCTimeZone which clearly does... UTC.

[douniwan5788]

A universal Time Field can be used everywhere. I thought the UTCTimeField meant that the value in this field is stored as UTC, and we can display it in a more useful local timezone. Should I rename the tz to astz to make it clearer?