gh-122449: Use constants instead of hard-coded strings in Tools/cases_generator

[picnixz]

This is a small PR for avoiding using hardcoded token's kind. By the way, I observed something weird: In always_exits of analyzer.py we have

cpython/Tools/cases_generator/analyzer.py

Lines 537 to 542 in d27a53f

	elif tkn.kind == "GOTO" or tkn.kind == "RETURN":
	return True
	elif tkn.kind == "KEYWORD":
	if tkn.text in EXITS:
	return True
	elif tkn.kind == "IDENTIFIER":

but a token kind set to "KEYWORD" does not exist (I added the constant but did not use it at all and I don't know how you could create one of that kind). I assume that the test should instead be something like tkn.kind in lx.keywords.values() instead but I would like confirmation on that matter.

I did not touch the kind of a Node since they are already literals (and mypy would then comply if something is wrong).

• Issue: Use constants instead of hard-coded strings in Tools/cases_generator #122449

[sobolevn]

I personally don't like star imports. What do you think about explicit names? Or import lexer and then lexer.NAME?

[picnixz]

Oh sorry, I should have used a draft PR for now. I actually wanted the star import to see which names are needed and then I'll change them one by one.

[picnixz]

I decided to:

• avoid changing the current imports and other usage
• always use import lexer as lx when many constants need to be imported. Then, I use them as lx.XXX.

[sobolevn]

Generally looks like a good refactoring, but I am not a code owner of this module, so I cannot make the full review.