Python: Trivially define the BOOST_PYTHON_EXPLICIT_TT_DEF macro

As BOOST_NO_EXPLICIT_FUNCTION_TEMPLATE_ARGUMENTS is never defined, this
macro is obsolete.
Python: Remove use of obsolete BOOST_PYTHON_EXPLICIT_TT_DEF macro.
Python: Simplify object and BOOST_PYTHON_FORWARD_OBJECT_CONSTRUCTORS macro.

The macro no longer needs an implementation for old MSVC.
Initial modular patch, from Bjorn.
Undo previous commit, it was meant for another branch
Remove use of obsolete BOOST_EXPLICIT_TEMPLATE_TYPE{,_SPEC} macros.
Remove use of obsolete BOOST_APPEND_EXPLICIT_TEMPLATE_TYPE{,_SPEC} macro.
Python: Simplify object_operators implementation.

The old-MSVC-specific version is no longer needed.
Python: Clean up some old MSVC related code.
Python: Remove obsolete MSVC version check.
Python: Remove obsolete GCC version checks.
Python: Fix build.