Support using prefixes with 0xff as the last byte.

[tmccombs]

This changes the logic for computing the range_end of a prefix so that
it works even if the prefix ends with one or more 0xff bytes

[codecov-io]

Codecov Report

Merging #1125 into master will decrease coverage by 0.20%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #1125      +/-   ##
==========================================
- Coverage   94.02%   93.82%   -0.21%     
==========================================
  Files          10       10              
  Lines         787      826      +39     
==========================================
+ Hits          740      775      +35     
- Misses         47       51       +4     

Impacted Files	Coverage Δ
etcd3/exceptions.py	88.88% <50.00%> (-11.12%)	⬇️
etcd3/locks.py	95.45% <94.28%> (-2.63%)	⬇️
etcd3/__init__.py	100.00% <100.00%> (ø)
etcd3/client.py	96.52% <100.00%> (+0.15%)	⬆️
etcd3/utils.py	100.00% <100.00%> (ø)
etcd3/watch.py	85.61% <0.00%> (-0.21%)	⬇️
etcd3/leases.py	100.00% <0.00%> (ø)
etcd3/members.py	81.25% <0.00%> (+1.25%)	⬆️

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[tmccombs]

The travis builds failed, but they are also failing on master. I haven't had much look getting the tests to run locally (I'm using python 3.8).

[tmccombs]

I haven't had much look getting the tests to run locally

Well besides the problem with dependencies that only work on python2, there's also this that was causing me problems

[kragniz]

thanks! Sorry for the delay in merging this, had to look at the CI and dep problems first