Restrict webargs versions

[saydamir]

webargs 6.0.0 has incompatible changes for example:

• Parser.use_args and Parser.use_kwargs now accept location (singular) instead of locations (plurala)

close: #176

[codecov-io]

Codecov Report

Merging #177 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #177   +/-   ##
=======================================
  Coverage   97.72%   97.72%           
=======================================
  Files           8        8           
  Lines         352      352           
=======================================
  Hits          344      344           
  Misses          8        8

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[sloria]

Thanks!