[2.7] bpo-31692: Add PYTHONSHOWALLOCCOUNT env var

[vstinner]

bpo-31692, bpo-19527:

• Add a new PYTHONSHOWALLOCCOUNT environment variable, similar to
  the Python 3 "-X showalloccount" option
• When Python is compiled with COUNT_ALLOCS, the new
  PYTHONSHOWALLOCCOUNT environment variable now has to be set to dump
  allocation counts into stderr on shutdown. Moreover, allocations
  statistics are now dumped into stderr rather than stdout.
• Add @test.support.requires_type_collecting decorator: skip test if
  COUNT_ALLOCS is defined
• Fix tests for COUNT_ALLOCS: decorate some methods with
  @requires_type_collecting
• test_sys.test_objecttypes(): update object type when COUNT_ALLOCS
  is defined

https://bugs.python.org/issue31692

[serhiy-storchaka]

Using the requires_type_collecting decorator LGTM. But I don't sure about adding PYTHONSHOWALLOCCOUNT. This looks like a new feature to me.

What if split this PR on two parts? The one fixes issues related to immortal types, the other is about additional output.

[serhiy-storchaka]

Using the requires_type_collecting decorator LGTM. But I don't sure about adding PYTHONSHOWALLOCCOUNT. This looks like a new feature to me.

What if split this PR on two parts? The one fixes issues related to immortal types, the other is about additional output.

[serhiy-storchaka]

It may be worth to document the changes in Doc/c-api/typeobj.rst too.

[vstinner]

I prefer to keep the description of all environment variables at the same place. I added a reference to the env var in Doc/c-api/typeobj.rst.

[vstinner]

Serhiy: "... This looks like a new feature to me ..."

I prefer to discuss on the bpo: https://bugs.python.org/issue31692#msg303959