mbed-cli doesn't work with python3 on Ubuntu 18.04

[yogpan01]

Ubuntu 18.04 now comes by default with python3 support and pip support is also through python3.
Using mbed-cli is now giving warning and sometimes weird failures failing to link the application because of some incompatible python code.

For example, I am trying to compile an application which requires to be built for ARMC6 but it fails in linking for me, even though I am using newer than 5.8 version of MbedOS.

 xxxx@ubuntu:~/example(master)$ mbed compile -t ARMC6 -m NUMAKER_PFM_M2351 --app-config configs/wifi_esp8266_v4.json
[mbed] WARNING: If you're using Python 3 with Mbed OS 5.8 and earlier versions, Python errors will occur when compiling, testing and exporting
---
[mbed] Working path "/home/yogpan01/example/mbed-client-example" (library)
[mbed] Program path "/home/yogpan01/example/mbed-client-example"
Building project mbed-client-example (NUMAKER_PFM_M2351, ARMC6)
Scan: mbed-client-lite-example-restricted
Link: mbed-client-lite-example-restricted
b"Warning: L3912W: Option 'legacyalign' is deprecated."
[ERROR] can't concat str to bytes
[mbed] ERROR: "/usr/bin/python3" returned error.
       Code: 1
       Path: "/home/xxxx/example/mbed-client-example"
       Command: "/usr/bin/python3 -u /home/yogpan01/example/mbed-client-example/mbed-os/tools/make.py -t ARMC6 -m NUMAKER_PFM_M2351 --source . --build ./BUILD/NUMAKER_PFM_M2351/ARMC6 --app-config configs/wifi_esp8266_v4.json"
       Tip: You could retry the last command with "-v" flag for verbose output
---
[mbed] WARNING: Using Python 3 with Mbed OS 5.8 and earlier can cause errors with compiling, testing and exporting
---

[screamerbg]

@yogpan01 Thanks for the bug report

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/MBOCUSTRIA-826

[cmonr]

@yogpan01 Cna you provide which version of Mbed OS you're using, as well the output of pip list?

[cmonr]

Also, as a rule of thumb, using verbose output when debugging is generally really helpful.

Would appreciate seeing the more verbose log.

[cmonr]

@yogpan01 @screamerbg Please retest this with a fresh Mbed OS master branch pull.

ARMmbed/mbed-os#9673 came in today which fixed a parsing error when using Py3 that looks like it was also affecting this PR.

If not, please provide more verbose output by also using the -v flag.

[yogpan01]

It works fine on MbedOS master, when we can have this fix for any mbed-os official release?

[vmedcy]

@yogpan01: ARMmbed/mbed-os#9673 targets 5.11.5 which is planned for the next week I guess.

[cmonr]

@yogpan01 @vmedcy is correct. The fix will be released with 5.11.5.

Additionally, looking at adding compile capabilities against all supported Python versions to make sure regressions aren't encountered, but not sure if this update will make it in time for 5.11.5.

Closing since issue is confirmed fixed and in master.

[cmonr]

@yogpan01 My mistake. Also saw the internal comment. Will keep open until 5.11.5 RC PR is created and merged.