Some Python 3 fixes

Some missing parentheses and one itertools.imap (Py2) vs map (Py3) issue.

gdb/ChangeLog:

	* python/lib/gdb/command/unwinders.py: Add parentheses.

gdb/testsuite/ChangeLog:

	* gdb.python/py-framefilter.py (ErrorFilter.filter): Use map function
	if itertools.imap is not present.
	* gdb.python/py-objfile.exp: Add parentheses.
	* gdb.python/py-type.exp: Same.
	* gdb.python/py-unwind-maint.py: Same.