Simplify json-to-dict utility

[theotherjimmy]

This should also resolve any missed ascii-ification bugs in python3

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1252
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6191/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Test : SUCCESS

Build number : 1049
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6191/1049

[mbed-ci]

Exporter Build : FAILURE

Build number : 916
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6191/

[0xc0170]

/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 924
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6191/

[adbridge]

This sits on top of the changes targeting 5.8 so moving to 5.8.1