Add soft keywords

[gvanrossum]

These are like keywords but they only work in context; they are not reserved except when there is an exact match.

This would enable things like match statements without reserving match (which would be bad for the re.match() function and probably lots of other places).

Automerge-Triggered-By: @gvanrossum

[gvanrossum]

Looks like the failing tests all have to do with print statements.

    test_exceptions test_future test_grammar test_print
    test_source_encoding

(UPDATE:)
And that was just a lark anyway (not meant seriously).

It does make me wonder if we shouldn't have a way to state that (certain) soft keywords should only be considered when the next character is a space.

[gvanrossum]

Fixed an unfortunate interaction with the new D() macro calls. Deleted the print statement easter egg. Now ready for review. (How to test this other than by adding a print statement?)

[pablogsal]

How to test this other than by adding a print statement?

A weak test would be adding something like this that uses the double quotes in the grammar and checks that _PyPegen_expect_soft_keyword is generated in the source and that something with single quotes in the grammar does not include _PyPegen_expect_soft_keyword in the source.

[gvanrossum]

Now with tests. I'd like to keep this as an easter egg, hence the skip issue label. No need to backport to 3.9 (unless you'd rather keep the source of Tools/peg_generator identical).

[pablogsal]

LGTM!

I left one question

[miss-islington]

@gvanrossum: Status check is done, and it's a success ✅ .