[yt-dev] Issue #972: Can't use attoseconds (yt_analysis/yt)

New issue 972: Can't use attoseconds https://bitbucket.org/yt_analysis/yt/issue/972/cant-use-attoseconds John ZuHone: Not really a big deal, but I noticed today you can't use attoseconds because prefixing "s" with "a" doesn't work: ```python yt.YTQuantity(1.0,"as") File "<string>", line 1 as ^ SyntaxError: invalid syntax ``` Responsible: jzuhone