pyisinstance error

```
pyisinstance(s, :PyDict_Type)
ERROR: MethodError: no method matching pyisinstance(::PyObject, ::Symbol)
```

I've just gone through the source code and found the following works:

```
pyisinstance(s, PyCall.@pyglobalobj :PyTuple_Type)
false
```

Can we have a ` pyisinstance(::PyObject, ::Symbol)` method exposed?

Start by reading the existing pyisinstance implementation and the source path where PyCall.@pyglobalobj is defined. Check how Python object and symbol arguments are currently handled, then add coverage for pyisinstance(s, :PyDict_Type) and confirm it produces the expected boolean result.

Scritto dal modello di indicizzazione a partire dal testo della issue.