Bug: save_signatures with a file opened in binary mode fails

[luizirber]

(Yes, it's one of those days)

This will fail in Python 3:

with open(output, "wb") as fp:
    sourmash.save_signatures([query], fp)

because there is a call to unicode(s) and unicode() doesn't exist in Python 3 anymore:

Traceback (most recent call last):
  File "src/sourmash/sourmash/signature_json.py", line 279, in write_records_to_json
    fp.write(s)
TypeError: a bytes-like object is required, not 'str'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "unassigned.py", line 54, in <module>
    sourmash.save_signatures([query], fp)
  File "/home/chick/Envs/2019-10-17-unassigned_hashes-gY7Uf7Ke/src/sourmash/sourmash/signature.py", line 262, in save_signatures
    return signature_json.save_signatures_json(siglist, fp)  File "/home/chick/Envs/2019-10-17-unassigned_hashes-gY7Uf7Ke/src/sourmash/sourmash/signature_json.py", line 298, in save_signatures_json    s = write_records_to_json(records, fp, indent, sort_keys)
  File "/home/chick/Envs/2019-10-17-unassigned_hashes-gY7Uf7Ke/src/sourmash/sourmash/signature_json.py", line 281, in write_records_to_json
    fp.write(unicode(s))
NameError: name 'unicode' is not defined

This PR adds a test to trigger the bug, and a fix that works in Py 3/2.

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]

Codecov Report

Merging #746 into master will increase coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #746      +/-   ##
==========================================
+ Coverage   88.28%   88.42%   +0.13%     
==========================================
  Files          30       30              
  Lines        4782     4786       +4     
  Branches       46       46              
==========================================
+ Hits         4222     4232      +10     
+ Misses        557      551       -6     
  Partials        3        3

Impacted Files	Coverage Δ
sourmash/signature_json.py	97.19% <100%> (+1.19%)	⬆️
sourmash/sbt_storage.py	85.39% <0%> (-0.98%)	⬇️
sourmash/sbt.py	85.53% <0%> (+0.31%)	⬆️
sourmash/lca/command_index.py	90.7% <0%> (+0.44%)	⬆️
sourmash/lca/lca_utils.py	96.01% <0%> (+0.79%)	⬆️

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[ctb]

LGTM! Let's merge when the tests on the updated merge pass.