Fix unnecessary SNI warning with SecureTransport

[hodbn]

Closes #1902.

[codecov]

Codecov Report

Merging #1903 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master     #1903      +/-   ##
===========================================
+ Coverage   99.95%   100.00%   +0.04%     
===========================================
  Files          23        23              
  Lines        2049      2052       +3     
===========================================
+ Hits         2048      2052       +4     
+ Misses          1         0       -1     

Flag	Coverage Δ
#unittests	99.56% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
src/urllib3/util/ssl_.py	100.00% <100.00%> (ø)
src/urllib3/util/wait.py	100.00% <0.00%> (+1.56%)	⬆️

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[sethmlarson]

Will give this a review today :)

[pquentin]

@sethmlarson Is there anything making reviewing this difficult?

[sethmlarson]

LGTM, sorry for taking so long on the review!