Do not use utc_now on module level

[sobolevn]

Looks like that this value is never used anywhere, and it should not be, because it is created once on a module import time.

I found this while working on python/typeshed#13514

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.70%. Comparing base (4b0701b) to head (3af0a8d).
Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
jose/jwt.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #372   +/-   ##
=======================================
  Coverage   92.69%   92.70%           
=======================================
  Files          15       15           
  Lines        1424     1439   +15     
=======================================
+ Hits         1320     1334   +14     
- Misses        104      105    +1     

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