fix: ignore ~/.netrc entries when authenticating to looker api

[roulettedares]

i had an older looker api entry in ~/.netrc which was causing me authentication headaches. this PR effectively ignores ~/.netrc entries for looker api. the official looker SDK does this as well https://github.com/looker-open-source/sdk-codegen/blob/master/python/looker_sdk/rtl/requests_transport.py#L77

[codecov]

Codecov Report

Merging #281 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #281      +/-   ##
==========================================
+ Coverage   91.97%   92.00%   +0.02%     
==========================================
  Files          17       17              
  Lines        1196     1200       +4     
==========================================
+ Hits         1100     1104       +4     
  Misses         96       96              

Impacted Files	Coverage Δ
spectacles/client.py	98.76% <100.00%> (+0.02%)	⬆️

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[joshtemple]

I'm a little wary of breaking support for credentials in .netrc files, but since it's never been something we've explicitly supported and since we already have a built in config file for Spectacles, I'm happy to merge this.