Fix python-shell-buffer-substring when retrieving a single statement

Fix python-shell-buffer-substring when retrieving a single statement

* lisp/progmodes/python.el (python-shell-buffer-substring): Do not add
"if True:" line when retrieving a single statement.
(python-shell-send-region): Add a reference to
`python-shell-buffer-substring' in docstring.
* test/lisp/progmodes/python-tests.el (python-shell-buffer-substring-13)
(python-shell-buffer-substring-14, python-shell-buffer-substring-15)
(python-shell-buffer-substring-16, python-shell-buffer-substring-17):
New tests. (Bug#60142)