[3.7] bpo-32962: Backport python-gdb.py and test_gdb.py from master (GH-7710)

[3.7] bpo-32962: Backport python-gdb.py and test_gdb.py from master (GH-7710)

* 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-32962: Fix test_gdb failure in debug build with -mcet -fcf-protection -O0 (GH-6754)

When Python is built with the intel control-flow protection flags,
-mcet -fcf-protection, gdb is not able to read the stack without
actually jumping inside the function. This means an extra
'next' command is required to make the $pc (program counter)
enter the function and make the stack of the function exposed to gdb.

(cherry picked from commit 9b7c74ca32d1bec7128d550a9ab1b2ddc7046287)