Fix version split for packages using --with-extra-version option

[marceloneppel]

This patch solves the error below when we have some extra data in the version string retrieved from pg_config output (like in PostgreSQL 12.9 (Ubuntu 12.9-0ubuntu0.20.04.1)):

...
File "...postgresql/installation.py", line 212, in __init__
    self.type, vs = self.version.split()
ValueError: too many values to unpack (expected 2)

As the extra data in the version string is added with the --with-extra-version option in the PostgreSQL configure script, like in the example command below, we can rely in getting just the two first parts of the version string without introducing a weird behavior for the already working version strings.

./configure --with-extra-version=" (Ubuntu 12.9-0ubuntu0.20.04.1)"

[jwp]

Seeing that configure option being builtin, I nearly committed this. However, it is, still, probably best to keep patches like this downstream as the next packaging system may not care to insert a space at all.