fix usage of Python GIL to make thread safe

[Vipul-Cariappa]

reference: root-project/root#19436
fixes/closes: wlav/cppyy#310

[wlav]

The DispatchPtr is only used internally from C++ by the Dispatcher. I wonder whether it wouldn't be more efficient to put the protections around its use rather than inside its methods.

[Vipul-Cariappa]

I forgot to wrap CPyCppyy::DispatchPtr::PythonOwns around the GIL.

[Vipul-Cariappa]

This is used in multiple places, other than the Dispatcher itself. Example:

CPyCppyy/src/CPPInstance.cxx

Lines 159 to 172 in 06986be

	void CPyCppyy::CPPInstance::PythonOwns()
	{
	fFlags |= kIsOwner;
	if ((fFlags & kIsExtended) && DISPATCHPTR(this))
	DISPATCHPTR(this)->PythonOwns();
	}
	//----------------------------------------------------------------------------
	void CPyCppyy::CPPInstance::CppOwns()
	{
	fFlags &= ~kIsOwner;
	if ((fFlags & kIsExtended) && DISPATCHPTR(this))
	DISPATCHPTR(this)->CppOwns();
	}

This implementation is simple, else we will need to search for all of the usages and wrap them around the GIL.
I guess in the above example, the GIL is held anyway.

Let me know your opinion. I can make modifications accordingly.