v1.13.2 — installer picks a supported Python

v1.13.2 — VisualWeaver

A fresh Mac or Linux install no longer fails on a too-new Python.

A new Homebrew or apt now installs Python 3.13 or 3.14, for which the pinned
PyMuPDF has no prebuilt wheel. The installer then tried to compile MuPDF from
source, which fails on current macOS SDKs.

install.sh now:

• picks the newest supported interpreter it can find — Python 3.11 or 3.12,
  including a Homebrew python@3.x that is not yet on PATH;
• honours PYTHON=/path/to/python3.12 ./install.sh as an explicit choice;
• refuses an unsupported Python up front, with the exact install command to run
  (brew install python@3.12 on macOS, apt-get install python3.12 python3.12-venv on Debian/Ubuntu) instead of a long source build that fails;
• rebuilds a virtualenv left behind on an unsupported Python rather than reusing
  it.

pip also refuses Python 3.13 and newer up front, and the documentation now
states the supported versions (3.11 or 3.12).