bpo-32962: Backport python-gdb.py and test_gdb.py from master (GH-7726)

bpo-32962: Backport python-gdb.py and test_gdb.py from master (GH-7726)

* bpo-32962: python-gdb catchs ValueError on read_var() (GH-7692)

python-gdb now catchs ValueError on read_var(): when Python has no
debug symbols for example.

(cherry picked from commit 019d33b7a447e78057842332fb5d3bad01922122)

* bpo-32962: python-gdb catchs UnicodeDecodeError (GH-7693)

python-gdb now catchs UnicodeDecodeError exceptions when calling
string().

(cherry picked from commit d22fc0bc7de7882da204abe50884bbde2da4f9e7)

bpo-29367: python-gdb.py now supports also method-wrapper
(wrapperobject) objects.

(cherry picked from commit 611083331d534481ca7956a376e38fb0e9ef3854)