stop using system Python to build wheels

[comex]

On Linux and Mac, pyflow intentionally runs python3 from PATH to build wheels, rather than using the copy of Python selected for the project:

// The Linux and Mac builds appear to be unable to build wheels due to
// missing the ctypes library; revert to system python.

This broke for me because of #177: the copy of Python I had installed as python3 didn't have wheel installed.

Even if that weren't an issue, it also happens to be a different version of Python from the one I selected for this project, so if I was building any packages that included C extensions, the wheels it built wouldn't work with the latter.

In general, using the system Python his seems like a workaround, but it should be replaced with a proper fix for whatever issue there is with ctypes.