Fix warnings always being displayed at library level

[jeremylow]

We were previously surfacing all warnings by setting warnings.simplefilter in the source of python-twitter itself. It seems like this is a decision best left to the end user whether and how (if they just want to see the warning or if they want to error out) they would like them handled. This removes that line of code.

Closes issue #403

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This change is 

[codecov-io]

Codecov Report

Merging #440 into master will decrease coverage by -0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #440      +/-   ##
==========================================
- Coverage   74.84%   74.83%   -0.02%     
==========================================
  Files           8        8              
  Lines        1944     1943       -1     
==========================================
- Hits         1455     1454       -1     
  Misses        489      489

Impacted Files	Coverage Δ
twitter/api.py	69.4% <ø> (-0.02%)	❌

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[bear]

Reviewed 1 of 1 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[bear]

+1 to merge even with the codecov fail - it's a one line deletion ... !?!