xgettext: In language Python, avoid stack overflow.

xgettext: In language Python, avoid stack overflow.

* gettext-tools/src/x-python.c (MAX_NESTING_DEPTH): New macro.
(paren_nesting_depth, bracket_nesting_depth): New variables.
(extract_balanced): Increase and check paren_nesting_depth or
bracket_nesting_depth before calling extract_balanced recursively.
(extract_python): Initialize paren_nesting_depth and bracket_nesting_depth.
* gettext-tools/tests/xgettext-python-stackovfl-1: New file.
* gettext-tools/tests/xgettext-python-stackovfl-2: New file.
* gettext-tools/tests/xgettext-python-stackovfl-3: New file.
* gettext-tools/tests/xgettext-python-stackovfl-4: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.