[MRG] make sourmash compatible with khmer 3

[luizirber]

Fixes load_nodegraph being removed from khmer 3.

(Travis is testing only on khmer 2.1.1, should we put a matrix config to check on both khmer 2 and 3? My vote is no.)

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov-io]

Codecov Report

Merging #508 into master will increase coverage by 0.14%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
+ Coverage    90.6%   90.75%   +0.14%     
==========================================
  Files          33       33              
  Lines        5002     5007       +5     
  Branches       36       36              
==========================================
+ Hits         4532     4544      +12     
+ Misses        470      463       -7

Impacted Files	Coverage Δ
sourmash/sbt.py	85.6% <50%> (-0.29%)	⬇️
sourmash/sbt_storage.py	95.5% <0%> (-1.09%)	⬇️
sourmash/lca/lca_utils.py	94.47% <0%> (+1%)	⬆️
sourmash/signature_json.py	95.51% <0%> (+1.92%)	⬆️
tests/test_signature_json.py	100% <0%> (+7.57%)	⬆️

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[luizirber]

Ready for review and merge @ctb