Fix memory leak in MinHash.intersection.

[camillescott]

The intersection function uses the new operator to construct
a KmerMinHash or KmerMinAbundance. Although this is Cython,
new still heap-allocates the object, and it is not marked
for garbage collection, because it isn't a PyObject. This commit
calls delete (del) on the allocated object to prevent it from
being leaked.

Addresses #685.

• 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]

Codecov Report

Merging #687 into master will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #687      +/-   ##
==========================================
- Coverage   89.26%   89.11%   -0.15%     
==========================================
  Files          27       27              
  Lines        4238     4237       -1     
  Branches       39       39              
==========================================
- Hits         3783     3776       -7     
- Misses        455      461       +6

Impacted Files	Coverage Δ
sourmash/signature_json.py	95.62% <0%> (-1.25%)	⬇️
sourmash/lca/lca_utils.py	95.23% <0%> (-0.8%)	⬇️
sourmash/lca/command_index.py	90.3% <0%> (-0.45%)	⬇️
sourmash/sbt.py	85.32% <0%> (-0.4%)	⬇️
sourmash/sbt_storage.py	86.36% <0%> (+0.97%)	⬆️

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