src/geoipyupdate/__init__.py: try os.replace() before shutil.move()

src/geoipyupdate/__init__.py: try os.replace() before shutil.move()

It turns out that shutil.move() is not atomic, which can lead to
issues when replacing the database under heavy load. There is an
alternative in os.replace() that *is* atomic, but does not work across
filesystems. For the best of both worlds, we now try os.replace(), and
fall back to shutil.move() if it fails.

Testing shows that os.replace() raises an OSError across multiple
filesystems, so that's what we catch. The documentation doesn't
specify one way or the other.

Thanks to Paulo Magalhães for report.