fp write for STIX Objects

[emmanvg]

Adding this piece of code would help write more flexible code while also providing the same options as serialize()

[emmanvg]

I started with having both under a same function, but I was doubtful about having two different behaviors (one that returned a string and one that returned None). Based on the same idea that json.dumps() and json.dump() are essentially the same except for the required fp argument and what each return, I thought it would be better to also keep them separate on stix2 too.

[codecov-io]

Codecov Report

Merging #500 (c2d360d) into master (f155e3e) will decrease coverage by 0.00%. The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master     #500      +/-   ##
==========================================
- Coverage   89.47%   89.47%   -0.01%     
==========================================
  Files         147      147              
  Lines       16554    16599      +45     
==========================================
+ Hits        14812    14852      +40     
- Misses       1742     1747       +5     

Impacted Files	Coverage Δ
stix2/base.py	74.70% <50.00%> (-0.20%)	:arrow_down:
stix2/datastore/filesystem.py	75.90% <66.66%> (-0.10%)	:arrow_down:
stix2/serialization.py	70.96% <71.42%> (-0.97%)	:arrow_down:
stix2/test/v20/test_bundle.py	100.00% <100.00%> (ø)
stix2/test/v21/test_bundle.py	100.00% <100.00%> (ø)
stix2/datastore/taxii.py	69.60% <0.00%> (-0.15%)	:arrow_down:
stix2/test/v20/test_datastore_taxii.py	100.00% <0.00%> (ø)
stix2/test/v21/test_datastore_taxii.py	100.00% <0.00%> (ø)

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[chisholm]

I didn't mean to combine them into one function. It was more like change the serialize() function to be something like:

with io.StringIO() as fp:
    fp_serialize(obj, fp)
    return fp.getvalue()

So the "main" code for serializing lives in one place, the two functions can't get out of sync, the serialize() implementation is shorter and simpler, etc. Just a thought.

[emmanvg]

I misinterpreted your prior message. It is a good suggestion...