simplejson -> json

[meejah]

I believe we discussed this at one nuts-and-bolts? I found this branch and it looks plausible (replacing all use of simplejson with the built-in json module).

[codecov-io]

Codecov Report

Merging #405 into master will increase coverage by 0.04%.
The diff coverage is 95.45%.

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
+ Coverage   87.96%   88.01%   +0.04%     
==========================================
  Files         145      145              
  Lines       27439    27439              
  Branches     3805     3805              
==========================================
+ Hits        24138    24150      +12     
+ Misses       2600     2590      -10     
+ Partials      701      699       -2

Impacted Files	Coverage Δ
src/allmydata/_auto_deps.py	91.66% <ø> (ø)	✅
src/allmydata/web/filenode.py	93.93% <100%> (ø)	✅
src/allmydata/web/introweb.py	100% <100%> (ø)	✅
src/allmydata/scripts/tahoe_ls.py	83.76% <100%> (ø)	✅
src/allmydata/web/directory.py	93.43% <100%> (-0.01%)	❌
src/allmydata/scripts/tahoe_manifest.py	90% <100%> (ø)	✅
src/allmydata/scripts/tahoe_check.py	80.91% <100%> (ø)	✅
src/allmydata/scripts/slow_operation.py	66.17% <100%> (ø)	✅
src/allmydata/web/status.py	83.14% <100%> (ø)	✅
src/allmydata/web/storage.py	96.77% <100%> (ø)	✅
... and 14 more

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[warner]

awesome, landing with a few comment updates