Make pyawaitable.h support precompiled headers.

[AraHaan]

This change makes it possible to have pyawaitable included inside of a precompiled header. Precompiled headers can sometimes make large C extensions compile faster so this change could benefit some users.

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[ZeroIntensity]

Could we add a test build for this? (See the PyAwaitable test package in tests/extension.)

[ZeroIntensity]

Let's hold off on this until we get the new build system working. We won't need pyawaitable_abi at all in the new vendored build.

[AraHaan]

I actually wonder if we should just make a new vendor.h that actually forward declares every function that will have a definition further down in the vendor file to where the end resulting file would be something similar to the style from Python.h on the new vendor branch.

[ZeroIntensity]

No need. The vendor script already handles everything.