lv2: use brewed python

[alebcay]

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This PR tackles a number of concerns with the lv2 formula.

• The build system (waf) was using macOS system Python (/usr/bin/env python) to run. Rewrite shebangs (and any included wscripts) to build using python@3.10
• One of the components that are shipped #{bin}/lv2specgen.py needs Python at runtime. It also needs several Python modules to be present. Create a virtualenv and include the necessary modules; rewrite shebangs to use the virtualenv Python. (AFAICT, this was broken previously, unless you happened to have the necessary dependencies installed where system Python can find them)
• Add a test block to check that the necessary Python modules are present, and to check the lv2 framework itself by building a small example plugin against it

[alebcay]

Also - considering that lv2 doesn't really need Python at runtime, but only lv2specgen, I'd be open to making an lv2specgen formula (especially seeing as some other lv2 dependents still seem to have transitive runtime dependency on python@3.9). Another option is to have lv2 use python@3.9 for now and then migrate it later, to avoid the audit conflict.

[BrewTestBot]

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