make sure ModelBuilder generates timezone aware values for Timestamptz

[dantownsend]

I realised that ModelBuilder wasn't generating timezone aware datetime values for Timestamptz columns. This can be important when testing.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (17e9448) 91.99% compared to head (62b542d) 91.98%.
Report is 2 commits behind head on master.

Files	Patch %	Lines
piccolo/apps/user/commands/list.py	71.42%	2 Missing ⚠️
piccolo/utils/printing.py	94.73%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #923      +/-   ##
==========================================
- Coverage   91.99%   91.98%   -0.01%     
==========================================
  Files         107      108       +1     
  Lines        8165     8195      +30     
==========================================
+ Hits         7511     7538      +27     
- Misses        654      657       +3     

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