build!: drop support for Python v3.7 and v3.8

[2bndy5]

Breaking Changes!

Python 3.7 and v3.8 have been dead for over a year now. v3.9 is slated for death in a few weeks. These changes still maintains support for v3.9 (or newer). I held out as long as I could, but this was inevitable.

• resolves deprecated license metadata #105
• bumps cibuildwheel to v3 and pybind11 to v3. This allows building/supporting Python v3.14.
• Revises CI workflows to satisfy some basic security concerns. Permissions are only explicitly granted when needed. Storage of git credentials are removed upon checkout.

Tip

The new craze in Python is free-threaded builds. Meaning, v3.14t (note the suffixed t in the python version) boasts true parallelism because it gets rid of the old GIL (mechanism that locks the python runtime to 1 thread).

We shouldn't need to do anything on our end because RF24 uses mutex locks for hardware access (to SPI buses, GPIO pins are guarded by the Linux Kernel's char-dev API). We only needed to bump the build tools (pybind11 and cibuildwheel) to support the newer CPython C API.

[2bndy5]

According to PiWheels, these are the versions of python shipped with each version of RPi OS

Raspberry Pi OS version	Python version
Buster	Python 3.7
Bullseye	Python 3.9
Bookworm	Python 3.11
Trixie	Python 3.13

This means we'd only be disrupting users who haven't updated their RPi machines in a long time (buster LTS ended in Jun 2024). At that point, I think those users would be expecting some obstacles if/when updating software.