Fix mypy error for field_for_type function. Fixes #672

[forsberg]

Description

An attempt to fix #672. I'm not all that experienced in using mypy/typing in Python, so there may be better ways.

#type: ignore is also an option, I guess.

[codecov-io]

Codecov Report

Merging #673 into master will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #673   +/-   ##
=======================================
  Coverage   98.32%   98.32%           
=======================================
  Files          99       99           
  Lines       10193    10195    +2     
  Branches     1139     1139           
=======================================
+ Hits        10022    10024    +2     
  Misses        138      138           
  Partials       33       33           

Impacted Files	Coverage Δ
faust/models/fields.py	97.26% <100.00%> (+<0.01%)	:arrow_up:
faust/models/record.py	96.39% <100.00%> (+0.01%)	:arrow_up:

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[camerondavison]

any progress on getting this pushed through? it could be nice to get venusian upgraded

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