PR: User environment variables to take precedence for IPython Console kernel

[mrclary]

Description of Changes

Ensure that user environment variables supersede Spyder runtime environment variables when starting the IPython Console kernel.

Issue(s) Resolved

Fixes #23711

[mrclary]

@spyder-ide/core-developers,
So I've run into an issue on this PR for Windows. The IPython console tests fail because a user-defined PATH variable clobbers the system-defined PATH variable. I see the same symptoms as #23582, so it may be related.

I'd like your feedback because I think we should clarify what get_user_environment_variables should do on all platforms.

Currently, get_user_environment_variables retrieves both "system" and "user" variables on posix systems, but only "user" variables on Windows. "User" variables will clobber "system" variables on all platforms except for PATH and PYTHONPATH on Windows. These seem to be an exception in which "system" and "user" values are concatenated. This is a problem when these are defined in the "system" variables and when using winreg to get user variables from HKEY_CURRENT_USER since the "system" values will not be retrieved.

In the specific case of this PR and possibly #23582, if the PATH environment variable sent to kernelspec is missing critical system paths for whatever reason, then the 'chcp' is not recognized as an internal or external command error will certainly result.

So should we also retrieve the environment variables from HKEY_LOCAL_MACHINE and concatenate the PATH and PYTHONPATH variable values? Note this would have implications for set_user_env.

[mrclary]

Also, should we consider using a subprocess script like we do for posix? Getting user (or system) variables from the registry does not resolve variable references, e.g. %SystemRoot%\system32 or %HOMEPATH%\some\directory.