virtual methods: garbage collected trampolines may corrupt future trampoline classes?

Issue description

When trying to investigate #1922, I accidentally reproduced the wrong thing, but ended up finding another bug. I haven't root-caused this, but this is the general breakdown:

• Define a base class + trampoline and bind those in C++, and then make derived class in Python which inherits from it.
• You should be able to instantiate both of them (if they are default-constructible)
• Delete those classes and garbage collect (and double-check to ensure they've been deleted)
• Define another base class + trampoline in C++, and then a derived class in Python
• Try constructing the new derived class; you may end up getting TypeError: __init__(self, ...) called with invalid `self` argument.

Reproducible example code

https://github.com/EricCousineau-TRI/pybind11/commit/c11472dec0e81b5f4842b54a1e19bae2f17f5773

I'm running this on Ubuntu 18.04, using Python 3.6.8.

My generic guess is that this is due to the use of hashing type.ptr(), which cause a hash collision when an object's memory location is reused after it's deleted (similar to #1922).

Start with the reproducible example in commit c11472dec0e81b5f4842b54a1e19bae2f17f5773 and follow the reported sequence of creating, deleting, and garbage-collecting trampoline classes. Confirm the failure when constructing the later Python-derived class, then identify and fix the cause so that construction remains valid after earlier classes are collected.