fix: when parsing python_strings options, don't fail if ' is last

Previously, if the last character was `'` parsing would fail: from `abc'`
it would produce the literal `'''abc''''`, which has one too many
unescaped single quote at the end.

This also fixes the issue for solo `'''`, where before it would produce
`''''\\'''''`, again with one too many single quote at the end.

I added tests for both cases.

kitty/conf/utils.py

@@ -127,6 +127,7 @@ def to_cmdline(x: str, expand: bool = True) -> list[str]:
 
 def python_string(text: str) -> str:
     from ast import literal_eval
+    text = (text[:-1] + "\\'") if text.endswith("'") else text
     ans: str = literal_eval("'''" + text.replace("'''", "'\\''") + "'''")
     return ans