Spellcheck PRs before merge.

[terryjreedy]

In spite of previous spelling fixes over the years, there have been, I recall, at least 3 spell fix PRs over the last several months. The problem is that we keep committing patches that add new doubled words and misspellings. For #6374, Guido, who does not like the code churn, said "Ideally spell checks would run on PRs.".

I presume people preparing the patches are using some combination of of python with re and some linux facility. I have no idea how many false positives such people have to ignore.

A separate issue is whether passing a spell check should be opt in or opt out.

[scop]

For my part, I've been using https://github.com/client9/misspell with decent results, as well as some local greps.