BUG: fix scipy.io.mmread() of gzipped files under Python3

fixes #2152

I have to admit I don't fully understand what's happening here. If I delete lines 459-461 and try to directly `fromstring(stream.read(), sep=' ')`, I get a `zlib.error`. Only after unsuccessfully trying `read()` once, I can `seek(0)` and then `read()`.  Any idea what's happening?

Apart from that, it works now on my system.