Wait & retry on rate limit errors for GitHub requests

[jwodder]

Closes #125.

[codecov-commenter]

Codecov Report

Merging #126 (e0c2700) into master (dc7aa53) will increase coverage by 0.88%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   71.33%   72.21%   +0.88%     
==========================================
  Files          10       10              
  Lines         928      961      +33     
  Branches      140      142       +2     
==========================================
+ Hits          662      694      +32     
- Misses        213      214       +1     
  Partials       53       53              

Impacted Files	Coverage Δ
src/tinuous/base.py	86.25% <38.46%> (-1.93%)	⬇️
src/tinuous/util.py	85.86% <50.00%> (-0.80%)	⬇️
src/tinuous/github.py	55.50% <72.50%> (+5.74%)	⬆️
src/tinuous/travis.py	66.36% <0.00%> (+2.72%)	⬆️

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[jwodder]

The test failure is because the version of git-annex in Ubuntu's repositories is too old for DataLad. #127 fixes that.

[yarikoptic]

This week result in up to about 50 sec sleep total with 10 retries. May be square it?

[jwodder]

Done.

[yarikoptic]

thanks!

[yarikoptic]

since no other pending PRs, let's just release. Thank you @jwodder!