Fix build_data functionality with python 3

[OPpuolitaival]

Description

Open file in non-binary mode because "The json module always produces str objects, not bytes objects. Therefore, fp.write() must support str input." https://docs.python.org/3.6/library/json.html#basic-usage

This found originally in here: https://circleci.com/gh/ARMmbed/mbed-cli/868

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Feature
[ ] Breaking change

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2810
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7769/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2438
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7769/

[mbed-ci]

Test : SUCCESS

Build number : 2545
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7769/2545