Tools: Fix py3 bug with mbed export -S

[theotherjimmy]

Description

I introduced a bug in #7410. This fixes it

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2577
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7481/

Triggering tests

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

[mbed-ci]

Exporter Build : FAILURE

Build number : 2220
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7481/

[mbed-ci]

Test : FAILURE

Build number : 2330
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7481/2330

[cmonr]

Exporter node left too soon.
/morph export-build

[cmonr]

And holding off on the re-test. Something funky is going on with tests atm.

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2228
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7481/

[cmonr]

/morph test

[mbed-ci]

Test : FAILURE

Build number : 2338
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7481/2338

[0xc0170]

Restarting the build

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2597
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7481/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2241
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7481/

[mbed-ci]

Test : SUCCESS

Build number : 2350
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7481/2350