Suppress Exceptions in keyring.get_password

[jaraco]

Fixes issue reported in #315.

[codecov]

Codecov Report

Merging #316 into master will increase coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
+ Coverage   67.42%   67.64%   +0.22%     
==========================================
  Files          12       12              
  Lines         574      578       +4     
  Branches       91       91              
==========================================
+ Hits          387      391       +4     
  Misses        162      162              
  Partials       25       25

Impacted Files	Coverage Δ
twine/utils.py	85.14% <100%> (+0.61%)	⬆️

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[brainwane]

I've given this a short review and it looks fine, and I'm inclined to trust you on this as you wrote keyring and keyring support in twine originally. :) Thanks.