A Possible Bug in Call Policies

Hi, I believe I just encountered a possible bug in boost-python...

First of all, the back story, I'm putting together a python-boost based example for my [`cmaketools`](https://pypi.org/project/cmaketools/0.1.3/) project, which makes a cmake-based python package build as straight-forward as possible. I've taken the examples by [TNG](https://github.com/TNG/boost-python-examples) (who created his examples based off of the official documentation) and turned them into a package with modules with pytest tests. The modules based on his "09-Overloading" example is failing "08-CallPolicies" example in pytest *only when they are run together*.

## Description

When there is a class with the same name but under different a module (in a same package?), `return_value_policy()` call (possibly?) picks a wrong class to instantiate under pytest running 2 test py files each testing one of the classes.

What is peculiar is that it works when only one test file is tested but fails when both are tested together. So, it could be a pytest issue (or Python's for that matter)...

## Versions

Boost: 1.71 (official pre-built binary)
Boost Python DLL: boost_python37-vc142-mt-x64-1_71.dll
Python: 3.7.6 (conda)
Compiler: MSVC 2019 (16.6.1)
OS: Window 10

## (Near) Minimal Example

Please find the entire example project at [https://github.com/hokiedsp/pytest_isinstance_issue](https://github.com/hokiedsp/pytest_isinstance_issue). Clone and run the following to reproduce (hopefully)

```bash
pip -e .
pytest tests/test_callpolicies.py # passes
pytest tests/test_nor.py # passes
pytest
```

Last pytest call fails with the assertion error:

```
def test_callpolicies():
f = policies.Example.factory()
> assert isinstance(f, policies.Example) and str(f) == "factory"
E AssertionError: assert (False)
E + where False = isinstance(, )
E + where = policies.Example
```

I'm anticipating `f` to be `boost_python_example.callpolicies.Example` but instead `boost_python_example.overloading.Example`.

Side Note: If CMake cannot find Boost, you need to modify Line 14 of `setup.py`

```python
configure_opts = ['-DBOOST_ROOT=C:\\Users\\tikum\\AppData\\Local\\Programs\\boost_1_71_0']
```

## code snippets

Here are the sections of most relevant:

`callpolicies.cpp`

```c++
#include
#include

class Example {
Example(std::string n) : mName(n) {}
std::string mName;
public:
std::string name() const { return mName; }
static Example* factory()
{
return new Example("factory");
}
// ... snip...
};

#include
using namespace boost::python;

BOOST_PYTHON_MODULE(callpolicies)
{
class_("Example", no_init)
.def("__str__", &Example::name)
.def("factory", &Example::factory,
return_value_policy())
.staticmethod("factory")
;
}
```

`overloading.cpp`

```c++
#include
#include

class Example {
public:
Example()
{}
// ... snip...
};

#include
using namespace boost::python;

BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS(makeIt_overloads, makeIt, 1, 3)
BOOST_PYTHON_MODULE(overloading)
{
void (Example::*d1)() = &Example::doit;
std::string (Example::*d2)(unsigned int) = &Example::doit;
void (Example::*d3)(std::string) = &Example::doit;

class_("Example")
.def("__str__", &Example::print)
// ...snip...
;
}
```

`test_callpolicies.py`

```python
#!/usr/bin/env python
import pytest
from boost_python_example import callpolicies as policies

def test_callpolicies():
# e = callpolicies.Example() # won't work, constructore not available from python
f = policies.Example.factory()
assert isinstance(f, policies.Example) and str(f) == "factory"
```