Problems with subprocess.py #9

Describe the bug
Trying to build QtWebEngine 6.3.2 (which moved to using python 3.x instead of python 2.x), I'm seeing the following error:
Traceback (most recent call last): File "U:/dev/qt6-webengine-os2-6.3.x/build/src/core/Release/i386/../../../../../src/3rdparty/chromium/third_party/devtools-frontend/src/third_party/typescript/ts_library.py", line 302, in <module> sys.exit(main()) File "U:/dev/qt6-webengine-os2-6.3.x/build/src/core/Release/i386/../../../../../src/3rdparty/chromium/third_party/devtools-frontend/src/third_party/typescript/ts_library.py", line 281, in main found_errors, stderr = runTsc( File "U:/dev/qt6-webengine-os2-6.3.x/build/src/core/Release/i386/../../../../../src/3rdparty/chromium/third_party/devtools-frontend/src/third_party/typescript/ts_library.py", line 56, in runTsc stdout, stderr = process.communicate() File "C:/USR/lib/python3.9/subprocess.py", line 1152, in communicate stdout, stderr = self._communicate(input, endtime, timeout) File "C:/USR/lib/python3.9/subprocess.py", line 2109, in _communicate stdout = b''.join(stdout) TypeError: sequence item 0: expected a bytes-like object, str found
To Reproduce
I will try and distill this down to a simple testcase.

Expected behavior
The command completes successfully. There are numerous matches of the final error - for example https://stackoverflow.com/questions/54863236/typeerror-sequence-item-0-expected-a-bytes-like-object-str-found