add 3.0 to test matrix

[jjb]

i checked a handful of other projects and they use a string instead of float, even when other versions use float, so i did the same here as well

[olleolleolle]

@jjb Hi! One of the very small things which can happen in YAML is a Float-to-String loss of characters.

An example (not from this project) of what it can look like before this change (well, before using '3.0'), in this picture:

Read more details, if you like: actions/runner#849

[olleolleolle]

Looks good to me!

[olleolleolle]

@jjb You inspired me to make this small change, too #9