Fix Python indentation of continuation lines within parens

Fix Python indentation of continuation lines within parens

* lisp/progmodes/python.el (python-indent-context): Add a new indent
context `:inside-paren-continuation-line'.
(python-indent--calculate-indentation): Use the new indent context.
* test/lisp/progmodes/python-tests.el (python-indent-pep8-2)
(python-indent-pep8-3)
(python-indent-inside-paren-1)
(python-indent-inside-paren-2)
(python-indent-inside-paren-3)
(python-indent-inside-paren-6)
(python-indent-after-backslash-2): Change to use the new indent
context.
(python-indent-inside-paren-8)
(python-indent-inside-paren-9): New tests. (Bug#63959)