hgext3rd/__init__.py

automation: set PATH when building on Windows

Sometime in the 6.2 release cycle the Windows building automation
broke. Building the wheel and even PyOxidizer based installers now fails
with:

```
Exception: PowerShell execution failed:   error: subprocess-exited-with-error
Getting requirements to build wheel did not run successfully.
exit code: 1
[1 lines of output]
Unable to find a working hg binary to extract the version from the repository tags
[end of output]
note: This error originates from a subprocess, and is likely not a problem with pip.
```

I have a hunch this is a regression from upgrading pip in 1c00777702da,
but I haven't verified this. It may not be, as PyOxidizer has its own
bundled Python/pip. So maybe it is something in `setup.py`.

# name space package to host third party extensions
import pkgutil

__path__ = pkgutil.extend_path(__path__, __name__)