Fix backports.zoneinfo import for Python<3.9

[codecov[bot]]

Codecov Report

Merging #80 (46280ba) into main (7cc4440) will decrease coverage by 0.05%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
- Coverage   79.04%   79.00%   -0.05%     
==========================================
  Files          32       32              
  Lines        2878     2881       +3     
==========================================
+ Hits         2275     2276       +1     
- Misses        603      605       +2     

Flag	Coverage Δ
unittests	79.00% <50.00%> (-0.05%)	:arrow_down:

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Files Changed	Coverage Δ
flow/record/fieldtypes/__init__.py	91.68% <50.00%> (-0.42%)	:arrow_down:

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