Fix Python 3.13 standalone mount steps

[mwaskom]

Older versions of the standalone Python distribution included only a python3 binary, so our Image recipe when using add_python added a softlink from python3 to python (which Modal otherwise expects to exist).

With newer versions of the standalone Python distribution, which we start picking up with Python 3.13, python is included so the build recipe would fail

=> Step 2: RUN ln -s /usr/local/bin/python3 /usr/local/bin/python
ln: failed to create symbolic link '/usr/local/bin/python': File exists

This PR skips the linking step based on a Python version check. (Technically it's the standalone mount version, not the Python version that matters, but the result is equivalent and the latter is easier to check in the relevant context).

[ekzhang]

Sounds good to me! Another approach is to just add || true to the end of the command, will let you pick which one is better though.

[mwaskom]

Sounds good to me! Another approach is to just add || true to the end of the command, will let you pick which one is better though.

Yeah — we'd need to do that conditionally anyway though right? If we just change the existing line it will force Image rebuilds I think.

[mwaskom]

or something like

add_python_commands = [
    "COPY /python/. /usr/local",
    "RUN ln -s /usr/local/bin/python3 /usr/local/bin/python" + (" || true" if python_version >= "3.13" else ""),
    "ENV TERMINFO_DIRS=/etc/terminfo:/lib/terminfo:/usr/share/terminfo:/usr/lib/terminfo",
]

kinda messy either way.