gh-85267: Improvements to inspect.signature __text_signature__ handling (#98796)

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 #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 #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 #85267), I
add constant folding of a couple binary operations to RewriteSymbolics.

(There's some discussion of making signature expression evaluation
arbitrary powerful in #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 #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`.

Lib/test/test_inspect.py

@@ -2525,7 +2525,7 @@ class TestSignatureObject(unittest.TestCase):
         self.assertEqual(p('f'), False)
         self.assertEqual(p('local'), 3)
         self.assertEqual(p('sys'), sys.maxsize)
-        self.assertNotIn('exp', signature.parameters)
+        self.assertEqual(p('exp'), sys.maxsize - 1)
 
         test_callable(object)
 
@@ -4323,10 +4323,29 @@ class TestSignatureDefinitions(unittest.TestCase):
         sig = inspect.signature(func)
         self.assertIsNotNone(sig)
         self.assertEqual(str(sig), '(self, /, a, b=1, *args, c, d=2, **kwargs)')
+
         func.__text_signature__ = '($self, a, b=1, /, *args, c, d=2, **kwargs)'
         sig = inspect.signature(func)
         self.assertEqual(str(sig), '(self, a, b=1, /, *args, c, d=2, **kwargs)')
 
+        func.__text_signature__ = '(self, a=1+2, b=4-3, c=1 | 3 | 16)'
+        sig = inspect.signature(func)
+        self.assertEqual(str(sig), '(self, a=3, b=1, c=19)')
+
+        func.__text_signature__ = '(self, a=1,\nb=2,\n\n\n   c=3)'
+        sig = inspect.signature(func)
+        self.assertEqual(str(sig), '(self, a=1, b=2, c=3)')
+
+        func.__text_signature__ = '(self, x=does_not_exist)'
+        with self.assertRaises(ValueError):
+            inspect.signature(func)
+        func.__text_signature__ = '(self, x=sys, y=inspect)'
+        with self.assertRaises(ValueError):
+            inspect.signature(func)
+        func.__text_signature__ = '(self, 123)'
+        with self.assertRaises(ValueError):
+            inspect.signature(func)
+
     def test_base_class_have_text_signature(self):
         # see issue 43118
         from test.ann_module7 import BufferedReader