[3.11] gh-85267: Improvements to inspect.signature __text_signature__ handling (GH-98796)

[hauntsaninja]

This makes a couple related changes to inspect.signature's behaviour when parsing a signature from __text_signature__.

First, inspect.signature is documented as only raising ValueError or TypeError. However, in some cases, we could raise RuntimeError. This PR changes that, thereby fixing GH-83685.

(Note that the new ValueErrors in RewriteSymbolics are caught and then reraised with a message)

Second, inspect.signature could randomly drop parameters that it didn't understand (corresponding to return None in the p function). This is the core issue in GH-85267. I think this is very surprising behaviour and it seems better to fail outright.

Third, adding this new failure broke a couple tests. To fix them (and to e.g. allow inspect.signature(select.epoll.register) as in GH-85267), I add constant folding of a couple binary operations to RewriteSymbolics.

(There's some discussion of making signature expression evaluation arbitrary powerful in GH-68155. I think that's out of scope. The additional constant folding here is pretty straightforward, useful, and not much of a slippery slope)

Fourth, while GH-85267 is incorrect about the cause of the issue, it turns out if you had consecutive newlines in text_signature, you'd get tokenize.TokenError.

Finally, the if name is invalid: code path was dead, since parse_name never returned invalid..
(cherry picked from commit 79311cb)

Co-authored-by: Shantanu [email protected]