Error with non-ASCII character?

[giomach]

Using 3.9.0 under Windows 10. I get an error with "pip install pyinstaller" as below (my username REDACTED in this excerpt):
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C:\Users\REDACTED>pip install pyinstaller
Collecting pyinstaller
Downloading pyinstaller-4.0.tar.gz (3.5 MB)
|████████████████████████████████| 3.5 MB 2.2 MB/s
Installing build dependencies ... done
Getting requirements to build wheel ... done
Preparing wheel metadata ... error
ERROR: Command errored out with exit status 1:
command: 'c:\users\REDACTED\appdata\local\programs\python\python39\python.exe' 'c:\users\REDACTED\appdata\local\programs\python\python39\lib\site-packages\pip\_vendor\pep517\_in_process.py' prepare_metadata_for_build_wheel 'C:\Users\CIARND~1\AppData\Local\Temp\tmp6k9s7407'
cwd: C:\Users\REDACTED\AppData\Local\Temp\pip-install-67epkggr\pyinstaller
Complete output (38 lines):
Error in sitecustomize; set PYTHONVERBOSE for traceback:
SyntaxError: (unicode error) 'utf-8' codec can't decode byte 0xe1 in position 0: unexpected end of data (sitecustomize.py, line 7)
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I suspect this is due to my username containing non-ASCII characters. The first such character in the username is "á", which in ANSI is 0xe1, and it looks like this is being treated as utf-8, where it would indeed be an error. (Elsewhere in this log, you see the username converted to Windows 8+3 form, but where I have redacted it, it appeared in full.)
Is there some setting to avoid this problem? Thanks.