gdb: fix flake8 warnings in gdb.base/gdb-index-many-types.py

Fix those:

    gdb/testsuite/gdb.base/gdb-index-many-types.py:17:18: F821 undefined name 'gdb'
    gdb/testsuite/gdb.base/gdb-index-many-types.py:26:42: F821 undefined name 'gdb'
    gdb/testsuite/gdb.base/gdb-index-many-types.py:29:16: F821 undefined name 'gdb'
    gdb/testsuite/gdb.base/gdb-index-many-types.py:31:19: F821 undefined name 'gdb'
    gdb/testsuite/gdb.base/gdb-index-many-types.py:33:16: F821 undefined name 'gdb'
    gdb/testsuite/gdb.base/gdb-index-many-types.py:33:51: F821 undefined name 'gdb'
    gdb/testsuite/gdb.base/gdb-index-many-types.py:47:17: E722 do not use bare 'except'

Change-Id: Iba1949a211af66e8dd1e6cb77a44845e5fa60c2e