tomllib: Limit number of parts of a key

[encukou]

Bug report

tomli has quadratic time complexity when handling parts of a key (via Flags.is_ calls in key_value_rule's loop, and maybe elsewhere). This isn't a problem unless a document has an excessive number (10_000-ish) of parts in a key.
Limiting input size seems like the best fix here.

Linked PRs

• gh-149231: tomllib: Limit the number of parts in a key #149233
• [3.15] gh-149231: tomllib: Limit the number of parts in a key (GH-149233) #149677
• [3.14] gh-149231: tomllib: Limit the number of parts in a key (GH-149233) #149815
• [3.15] gh-149231: Revert extra NEWS entry #149840
• [3.13] gh-149231: tomllib: Limit the number of parts in a key (GH-149233) (GH-149815) #149848
• [3.12] gh-149231: tomllib: Limit the number of parts in a key (GH-149233) (GH-149815) #154143