Stringify infinity in JSON output.

[grahamgower]

This does require explicit conversion of string-valued "Infinity" when
loading the JSON file, but that code is quite modest.
See popsim-consortium/demes-spec#70

[codecov]

Codecov Report

Merging #311 (a3e7b71) into main (e5619c5) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #311      +/-   ##
==========================================
+ Coverage   98.12%   98.21%   +0.08%     
==========================================
  Files           8        8              
  Lines        1549     1570      +21     
==========================================
+ Hits         1520     1542      +22     
+ Misses         29       28       -1     

Impacted Files	Coverage Δ
demes/load_dump.py	100.00% <100.00%> (ø)
demes/hypothesis_strategies.py	98.63% <0.00%> (+0.68%)	⬆️

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[jeromekelleher]

LGTM, minor comments

[grahamgower]

Thanks @jeromekelleher. All good points, which are now resolved.