fix: formatting nanoseconds to Flux AST

[bednar]

Closes #455

Proposed Changes

Fixed formatting datetime with nanoseconds to Flux AST.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• pytest tests completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Merging #457 (0897712) into master (22a8707) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #457   +/-   ##
=======================================
  Coverage   89.34%   89.35%           
=======================================
  Files          38       38           
  Lines        3258     3260    +2     
=======================================
+ Hits         2911     2913    +2     
  Misses        347      347           

Impacted Files	Coverage Δ
influxdb_client/client/_base.py	97.84% <100.00%> (+0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 22a8707...0897712. Read the comment docs.

[rhajek]

LGTM