fix regression discussed by @cyfdecyf in #1454

[wjakob]

The PYBIND11_TLS_DELETE_VALUE in PR #1454 changed behavior wrt. Python 2.x. This commit restores the old behavior.

[wjakob]

@cyfdecyf: I was not able to reproduce the issue when calling pure virtual functions multiple times in Python 2.7.x, thus it's hard for me to say if this PR addresses the issue. Could you please give it a try? For the record, it would also be great to know what OS you're running this on.

[cyfdecyf]

Here's a simple test case. I can confirm this commit fixes the problem.

Note the problem only occurs when gil_scope_acquire is called in a non-python created thread, thus only test_in_new_thread will trigger the bug.

Without this fix, test.py will segfault in gil_scope_acquire in the second lock because it's getting the already deleted TLS tstate which is allocated in the 1st call.

Environment:
Ubuntu 16.04.4
Python 2.7.12

[wjakob]

Thank you @cyfdecyf, I added your test.