Support cloud-client 0.11.x

[randomir]

In this PR we:

• increase the cloud-client upper bound to 0.12.0
• test with 0.11.0.dev0
• fix docs build for edge case of running tests with a pre-release version of cloud-client that's not yet supported by dwave-system

[codecov[bot]]

Codecov Report

Merging #500 (79565e1) into master (11a50ba) will decrease coverage by 3.17%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #500      +/-   ##
==========================================
- Coverage   90.87%   87.70%   -3.17%     
==========================================
  Files          24       24              
  Lines        1676     1675       -1     
==========================================
- Hits         1523     1469      -54     
- Misses        153      206      +53     

Files	Coverage Δ
dwave/system/samplers/dwave_sampler.py	84.06% <ø> (-2.82%)	:arrow_down:

... and 5 files with indirect coverage changes

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