Python freshly reinstalled to C:\Program Files\Python39, everything done as administrator, installed for all users, added to PATH.
Error in installing pyinstaller.
My Windows username contains the character á (0xe1 in ANSI), which is being misinterpreted as being in utf8 instead of ANSI, whence error.
There is no sitecustomize.py in my file system.

Microsoft Windows [Version 10.0.19041.572]
(c) 2020 Microsoft Corporation. All rights reserved.

C:\WINDOWS\system32>python
Python 3.9.0 (tags/v3.9.0:9cf6752, Oct  5 2020, 15:34:40) [MSC v.1927 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()

C:\WINDOWS\system32>python -V
Python 3.9.0

C:\WINDOWS\system32>py -m pip install --upgrade pip
Requirement already up-to-date: pip in c:\program files\python39\lib\site-packages (20.2.4)

C:\WINDOWS\system32>py -m pip install pyinstaller
Collecting pyinstaller
  Using cached pyinstaller-4.0.tar.gz (3.5 MB)
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... error
    ERROR: Command errored out with exit status 1:
     command: 'C:\Program Files\Python39\python.exe' 'C:\Program Files\Python39\lib\site-packages\pip\_vendor\pep517\_in_process.py' prepare_metadata_for_build_wheel 'C:\Users\CIARND~1\AppData\Local\Temp\tmpm0upp60n'
         cwd: C:\Users\Ciarán Ó Duibhín\AppData\Local\Temp\pip-install-jw1e9dw4\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 21)
    running dist_info
    creating C:\Users\Ciarán Ó Duibhín\AppData\Local\Temp\pip-modern-metadata-48slrgbm\pyinstaller.egg-info
    writing C:\Users\Ciarán Ó Duibhín\AppData\Local\Temp\pip-modern-metadata-48slrgbm\pyinstaller.egg-info\PKG-INFO
    writing dependency_links to C:\Users\Ciarán Ó Duibhín\AppData\Local\Temp\pip-modern-metadata-48slrgbm\pyinstaller.egg-info\dependency_links.txt
    writing entry points to C:\Users\Ciarán Ó Duibhín\AppData\Local\Temp\pip-modern-metadata-48slrgbm\pyinstaller.egg-info\entry_points.txt
    writing requirements to C:\Users\Ciarán Ó Duibhín\AppData\Local\Temp\pip-modern-metadata-48slrgbm\pyinstaller.egg-info\requires.txt
    writing top-level names to C:\Users\Ciarán Ó Duibhín\AppData\Local\Temp\pip-modern-metadata-48slrgbm\pyinstaller.egg-info\top_level.txt
    writing manifest file 'C:\Users\Ciarán Ó Duibhín\AppData\Local\Temp\pip-modern-metadata-48slrgbm\pyinstaller.egg-info\SOURCES.txt'
    reading manifest file 'C:\Users\Ciarán Ó Duibhín\AppData\Local\Temp\pip-modern-metadata-48slrgbm\pyinstaller.egg-info\SOURCES.txt'
    reading manifest template 'MANIFEST.in'
    warning: no files found matching 'pyinstaller-gui.py'
    no previously-included directories found matching 'bootloader\build'
    no previously-included directories found matching 'bootloader\.waf-*'
    no previously-included directories found matching 'bootloader\.waf3-*'
    no previously-included directories found matching 'bootloader\waf-*'
    no previously-included directories found matching 'bootloader\waf3-*'
    no previously-included directories found matching 'bootloader\_sdks'
    no previously-included directories found matching 'bootloader\.vagrant'
    warning: no previously-included files found matching 'bootloader\.lock-waf*'
    no previously-included directories found matching 'doc\source'
    no previously-included directories found matching 'doc\_build'
    warning: no previously-included files matching '*.tmp' found under directory 'doc'
    warning: no files found matching 'news\_template.rst'
    no previously-included directories found matching 'news'
    no previously-included directories found matching 'old'
    no previously-included directories found matching 'scripts'
    no previously-included directories found matching 'tests\scripts'
    no previously-included directories found matching '.github'
    warning: no previously-included files found matching '.*'
    warning: no previously-included files found matching '*.yml'
    warning: no previously-included files found matching '*~'
    warning: no previously-included files found matching '.directory'
    warning: no previously-included files matching '*.py[co]' found anywhere in distribution
    writing manifest file 'C:\Users\Ciarán Ó Duibhín\AppData\Local\Temp\pip-modern-metadata-48slrgbm\pyinstaller.egg-info\SOURCES.txt'
    creating 'C:\Users\Ciarán Ó Duibhín\AppData\Local\Temp\pip-modern-metadata-48slrgbm\pyinstaller.dist-info'
    error: invalid command 'bdist_wheel'
    ----------------------------------------
ERROR: Command errored out with exit status 1: 'C:\Program Files\Python39\python.exe' 'C:\Program Files\Python39\lib\site-packages\pip\_vendor\pep517\_in_process.py' prepare_metadata_for_build_wheel 'C:\Users\CIARND~1\AppData\Local\Temp\tmpm0upp60n' Check the logs for full command output.

C:\WINDOWS\system32>
C:\WINDOWS\system32>