add Scala 3.0.0-M3 (drop M2)

[SethTisue]

No description provided.

[SethTisue]

it isn't this PR's fault, but as we've also seen on recent 1.2.x branch builds (example: https://travis-ci.com/github/scala/scala-parser-combinators/builds/209505546), one job in this PR's build, https://travis-ci.com/github/scala/scala-parser-combinators/jobs/463087281, is failing. sbt fails to launch with

java.lang.NullPointerException
at sun.net.util.URLUtil.urlNoFragString(URLUtil.java:50)
at sun.misc.URLClassPath.getLoader(URLClassPath.java:522)

I have no idea what the cause of this might be.

It seems like it could be something bad in cache:

attempting to download cache archive
fetching PR.335/cache--linux-xenial-05eeeb8559ec25b582c4f865e35e5d10e2d12ce8a2b5570e12da2728da24bc1b--jdk-default--scala-2.13.4.tgz
fetching PR.335/cache-linux-xenial-05eeeb8559ec25b582c4f865e35e5d10e2d12ce8a2b5570e12da2728da24bc1b--jdk-default--scala-2.13.4.tgz
fetching PR.335/cache--jdk-default--scala-2.13.4.tgz
fetching 1.2.x/cache--linux-xenial-05eeeb8559ec25b582c4f865e35e5d10e2d12ce8a2b5570e12da2728da24bc1b--jdk-default--scala-2.13.4.tgz
found cache

so logically I would like to clear the cache and try again, but when I visit https://travis-ci.com/github/scala/scala-parser-combinators/caches, it says "No caches have been created yet", which appears to be incorrect, based on the above log snippet

[SethTisue]

I worked around it with a dummy environment variable. PR to remove it again is #336