Python 3.9 freshly installed in Windows 10 to C:\Program Files\Python39, everything done as administrator, installed for all users, added to PATH.

Installation of pyinstaller fails with message "SyntaxError: (unicode error) 'utf-8' codec can't decode byte 0xe1 in position 0: unexpected end of data (sitecustomize.py, line 21)" (attachment)

This may be due to the character á in my Windows username (0xe1 in ANSI, which is my default 8-bit codepage) being misinterpreted as being in utf8 instead of ANSI.

There is no sitecustomize.py in my file system.

This looks to be a bug in the constructor of pip's BuildEnvironment class:

https://github.com/pypa/pip/blob/a4f4bfbf8ba7fd1e60884a439907e3f2a32e117a/src/pip/_internal/build_env.py#L82

Python 3 source files are UTF-8 by default, but text files default to the platform's preferred encoding, unless overridden by Python's UTF-8 mode. It is thus a mistake to create "sitecustomize.py" without explicitly specifying UTF-8 as the encoding. 

The preferred encoding in Windows is the process active code page, which is the system code page, unless, in Windows 10 only, it's overridden to use UTF-8 by the application manifest's "activeCodePage" setting. The system code page is usually a legacy code page, such as 1252, unless, in Windows 10 only, it's set to UTF-8 or the system language is Unicode only (e.g. Hindi).

A 'crash' is when the program hangs or stops unexpectedly *without* an Exception and message.

Lib/site-packages/sitecustomize.py is an optional added module.
https://docs.python.org/3/library/site.html#index-5
Apparently, attempted installation of pyinstaller resulted in an attempt to add it.  Or it was later attempting to read a badly written file, and removed it when cleaning up.

In any case, this appears to be a 3rd party bug that should be reported on the pypa/pip tracker.  Paul, are you involved with pip?  If so, can you add anything?

Erik is correct, this is a pip bug. Please raise an issue on the pip tracker linking to this issue and we can fix it.

I'm not sure whose responsibility it is to take this to the "pip tracker", but as a newcomer to the python environment I know nothing about that process, so it would be appreciated if some of the other people here would be kind enough to do this.

I've raised https://github.com/pypa/pip/issues/9135 for this.