Correctly use gnu-sed on macos

[ssbarnea]

• corrects gnu-sed installation instructions on macos, avoids asking to alter PATH as this might cause problems with other tools, including system ones
• uses gsed on macos
• display error message if gnu-sed is missing on macos

Related: #68

[ssbarnea]

@asottile If desired I could try to activate a macos job, so we can avoid the no pragma. Also, I observe that last two versions of python are not even tested. Just let me know if you need a hand.

[asottile]

this has already been rejected before, please search closed PRs

[ssbarnea]

With all due respect, i did search both issues and PRs containing 'sed', including closed/rejected ones and I did not find any indication that there is no desire to make it work on macos in a non intrusive way.

The closest thing I found was #137 (comment) which is a comment from you that makes me believe you would want to test for the missing or wrong sed on macos.

PS. 10min later of fully reading all the comments on ~15 tickets and I found a deep comment that 3y old #184 (comment) -- that explains it. Still, I guess you are fully aware that the README.md documentation about prepending gnu sed to PATH is high likely that will break other tools. -- So sorry for trying to improve it.