Startup deadlock when using multiple interpreters in same process

Issue description

When multiple sub-interpreters are created in the same process, the importing of a module created using pybind11 will cause a deadlock, as the GIL for the wrong interpreter is locked. mod_wsgi is the largest example app which will create multiple sub-interpreters, and describe the issue here.

This issue occurs only with pybind11 >= 2.4.0. Older versions do not have this issue (at least as far back as 2.2.3)

Reproducible example code

Code difficult to provide without dependencies, as this is a rather specific integration bug. Most simple way to reproduce is:

1. Install Apache httpd with mod_wsgi
2. Create multiple WSGIScriptAlias definitions in apache config file, both pointing to a script which imports a module created with pybind11
3. At least 1 interpreter will deadlock on import statement.

Attaching the the process reveals that the deadlock occurs here.
Use of PyGILState_Ensure will cause issues in processes with multiple interpreters. If this section is replaces with equivalent code to use of Py_BEGIN_ALLOW_THREADS and Py_END_ALLOW_THREADS, no issue should occur, as these APIs correctly deal with multiple sub-interpreters.

See reference here for more info on the issue. Specifically, see note of:

Note that the PyGILState_*() functions assume there is only one global interpreter (created automatically by Py_Initialize()). Python still supports the creation of additional interpreters (using Py_NewInterpreter()), but mixing multiple interpreters and the PyGILState_*() API is unsupported.

This implies that the full example code above should be used, instead of PyGILState_*

Start at include/pybind11/detail/internals.h around line 240 and review the GIL handling used during module import. Compare the current PyGILState_* usage with the Py_BEGIN_ALLOW_THREADS/Py_END_ALLOW_THREADS path described in the issue, then validate with multiple sub-interpreters through mod_wsgi. Done means imports no longer deadlock across sub-interpreters.