Replace removed sysconfig._is_python_source_dir with equivalent

[virtuald]

For #103 , but Python 3.11 still doesn't work. Tested with 3.11 final release. Heres the errors I'm still getting.

# python3.11 -m crossenv /build/crosspy/bin/python3.11 /build/venv --sysroot=$(arm-frc2023-linux-gnueabi-gcc -print-sysroot) --env UNIXCONFDIR=/build/venv/cross/etc -vvvv
WARNING: CC is a compound command (['arm-frc2023-linux-gnueabi-gcc', '-pthread'])
WARNING: This can cause issues for modules that don't expect it.
WARNING: Consider setting CC='arm-frc2023-linux-gnueabi-gcc' and CFLAGS='-pthread'
WARNING: CXX is a compound command (['arm-frc2023-linux-gnueabi-c++', '-pthread'])
WARNING: This can cause issues for modules that don't expect it.
WARNING: Consider setting CXX='arm-frc2023-linux-gnueabi-c++' and CXXFLAGS='-pthread'
WARNING: The cross-compiler ('arm-frc2023-linux-gnueabi-gcc -pthread') does not appear to be for the correct architecture (got arm-nilrt-linux-gnueabi, expected armv7l-frc2023-linux-gnueabi). Use --cc to correct, if necessary.
INFO: Creating build-python environment
INFO: Creating cross-python environment
INFO: Installing cross-pip
DEBUG: Installing: ['pip==22.3', 'setuptools==65.5.0']
/build/venv/cross/bin/python3.11: No module named pip
INFO: Finishing up...

I looked at it for awhile and... I'm not really sure what's going on. I'm open to looking into this more if you can give me some hints.

---

Figured it out. Fixes pushed here.

[Safihre]

Thank you, this works! We rely heavily on crossenv at SynoCommunity.

[Safihre]

Any updates? Hoping for a Python 3.11 compatibility release 😊

[virtuald]

Changes made. FWIW my project has been using this for a month and it works great as far as I can tell.

[benfogle]

Looks good. Merging now.

[Safihre]

Awesome 👍👍