Can AT&T internet subscribers access Astros games?

You have to be subscribed to whatever network is televising the game (ie. ESPN).   Unless you are subscribed through another provider like DirecTV or Spectrum, then no.  They don't care what ISP you are using.

@skeeterintexas 

I thought the TV and internet providers had to "match" i.e. both be AT&T, Comcast, whatever.

For example, U-Verse internet and Spectrum TV wouldn't get the streaming, but U-Verse for both would.

...or is that no longer the case? I believe it was at one time.

I don't know the answer to that.  I just thought that a cable only network (ESPN, NHL, etc.) would only care if you are a subscriber to their network since that's where the dollars are.  But I've never had TV service with one provider and internet with different provider.