Rationale: Not interpreting $ and \ in the here documents is a better default
than interpreting them.
* gettext-tools/tests/*: Write 'cat <<\EOF' instead of 'cat <<EOF', when no
shell dollar-substitution and backslash-interpretation is needed.
* 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.
