bpo-29564:_PyMem_DumpTraceback() suggests enabling tracemalloc

[vstinner]

If tracemalloc is not tracing Python memory allocations,
_PyMem_DumpTraceback() now suggests to enable tracemalloc to get the
traceback where the memory block has been allocated.

https://bugs.python.org/issue29564

[vstinner]

Example:

$ ./python -c 'import _testcapi; _testcapi.pymem_buffer_overflow()'
Debug memory block at address p=0x7f1d77a7a160: API 'm'
    16 bytes originally requested
    The 7 pad bytes at p-7 are FORBIDDENBYTE, as expected.
    The 8 pad bytes at tail=0x7f1d77a7a170 are not all FORBIDDENBYTE (0xfb):
        at tail+0: 0x78 *** OUCH
        at tail+1: 0xfb
        at tail+2: 0xfb
        at tail+3: 0xfb
        at tail+4: 0xfb
        at tail+5: 0xfb
        at tail+6: 0xfb
        at tail+7: 0xfb
    The block was made by call #28224 to debug malloc/realloc.
    Data at p: cb cb cb cb cb cb cb cb cb cb cb cb cb cb cb cb

Enable tracemalloc to get the memory block allocation traceback

Fatal Python error: bad trailing pad byte

Current thread 0x00007f1d77b625c0 (most recent call first):
  File "<string>", line 1 in <module>
Aborted (core dumped)

The change adds "Enable tracemalloc to get the memory block allocation traceback" line.

See also: https://pythondev.readthedocs.io/debug_tools.html#memory-debugger

[miss-islington]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[bedevere-bot]

GH-10514 is a backport of this pull request to the 3.7 branch.

[miss-islington]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.6.
🐍🍒⛏🤖

[bedevere-bot]

GH-10515 is a backport of this pull request to the 3.6 branch.