Issue 34537: test_gdb fails with LC_ALL=C

On Python 3.7 and master, test_strings() of test_gdb fails if gdb uses Python 3.6 (before UTF-8 Mode and C locale coercion):

* gdb uses ASCII encoding for stdout
* tested Python uses UTF-8 thanks to the UTF-8 Mode and/or C locale coercion


vstinner@apu$ LC_ALL=C ./python -m test -v test_gdb -m test_strings
== CPython 3.7.0+ (heads/3.7:65ef7425a3, Aug 28 2018, 16:29:30) [GCC 8.1.1 20180712 (Red Hat 8.1.1-5)]
== Linux-4.17.6-200.fc28.x86_64-x86_64-with-fedora-28-Twenty_Eight little-endian
== cwd: /home/vstinner/prog/python/3.7/build/test_python_21689
== CPU count: 8
== encodings: locale=UTF-8, FS=utf-8
Run tests sequentially
0:00:00 load avg: 0.35 [1/1] test_gdb
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test_strings (test.test_gdb.PrettyPrintTests)
Verify the pretty-printing of unicode strings ... FAIL

======================================================================
FAIL: test_strings (test.test_gdb.PrettyPrintTests)
Verify the pretty-printing of unicode strings
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/prog/python/3.7/Lib/test/test_gdb.py", line 331, in test_strings
    check_repr('\u2620')
  File "/home/vstinner/prog/python/3.7/Lib/test/test_gdb.py", line 323, in check_repr
    self.assertGdbRepr(text)
  File "/home/vstinner/prog/python/3.7/Lib/test/test_gdb.py", line 271, in assertGdbRepr
    % (gdb_repr, exp_repr, gdb_output)))
AssertionError: "'\\u2620'" != "'☠'"
- '\u2620'
+ '☠'
 : "'\\u2620'" did not equal expected "'☠'"; full output was:
Breakpoint 1 at 0x52fc1b: file Python/bltinmodule.c, line 1216.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Breakpoint 1, builtin_id (self=, v='\u2620') at Python/bltinmodule.c:1216
1216	    return PyLong_FromVoidPtr(v);
#0  builtin_id (self=, v='\u2620') at Python/bltinmodule.c:1216


----------------------------------------------------------------------
Ran 1 test in 1.030s

FAILED (failures=1)
test test_gdb failed
test_gdb failed

== Tests result: FAILURE ==

1 test failed:
    test_gdb

Total duration: 1 sec 422 ms
Tests result: FAILURE

New changeset 7279b5125e7c5d84a473d250b27d353cb7f6628e by Victor Stinner (Elvis Pranskevichus) in branch 'master':
bpo-34537: Fix test_gdb:test_strings with LC_ALL=C (GH-9483)
https://github.com/python/cpython/commit/7279b5125e7c5d84a473d250b27d353cb7f6628e

I don't think that it's worth it to backport the change to Python 3.6 and older, since "LC_ALL=C ./python -m test test_gdb" already pass on 2.7 and 3.6.

New changeset e5fde1f992e94f166415ab96d874ed1d2e0c8004 by Miss Islington (bot) in branch '3.7':
bpo-34537: Fix test_gdb:test_strings with LC_ALL=C (GH-9483)
https://github.com/python/cpython/commit/e5fde1f992e94f166415ab96d874ed1d2e0c8004

Thank you Elvis for the fix!