Version of installed pip.

def pip_version(self):
        """Version of installed pip."""
        if not self._pip_exists:
            return None
        if not hasattr(self, '_pip_version'):
            # don't call `self._execute_pip` here as that method calls this one
            output = self._execute(self._pip + ['-V'], log=False).split()[1]
            self._pip_version = tuple([int(n) for n in output.split('.')])
        return self._pip_version