Avoid building pyrobuf extensions during metadata

[rlopezquadient]

This fixes source installs of pyrobuf with modern setuptools / pip build isolation.

Root cause :
During metadata generation, setup.py was triggering builtin extension generation too early through the custom Distribution flow. That caused setuptools metadata preparation to execute build-time side effects, and source installs failed with:

AttributeError: 'PyrobufDistribution' object has no attribute 'dry_run'

[juntyr]

An alternative and much simpler fix is to add a constraint to setup_requires that bounds setuptools < 81

[Kevin-Prichard]

This fixes source installs of pyrobuf with modern setuptools / pip build isolation.
...
An alternative and much simpler fix is to add a constraint to setup_requires that bounds setuptools < 81

Either way it's happening in the wild, with users experiencing this breakage on pip install pyrobuf and its dependents, e.g. PyrOSM.

Please choose a fix and release