internals::tstate is leaked

[bkloster-sma]

Issue description

The Py_tss_t* internals::tstate created in get_internals() with PyThread_tss_create() is never released with PyThread_tss_delete(). This is usually not a problem, unless you happen to initialize an interpreter in a loop repeatedly. Eventually, the PyThread_tss_create() will fail (on Windows, because TlsAlloc() returns TLS_OUT_OF_INDEXES), causing get_internals() to throw an exception with the message "get_internals: could not successfully initialize the TSS key!".

On my machine, this happens when initializing the interpreter for the 1070th time.

Reproducible example code

while (true) {
    pybind11::initialize_interpreter();
    pybind11::finalize_interpreter();
}

[bstaletic]

#2020