[Python] Remove / Update code that might not be required with Cython > 2

[raulcd]

Describe the enhancement requested

When bumping Cython to >= 3, on:

• [Python] Raise minimum supported cython to >=3  #45237

We noticed there's a couple of places where there are comments related to some code required in order to make it compatible with both Cython 2 and 3:
#45238 (comment)

This can possibly be removed or simplified.

Basically:

arrow/python/pyarrow/includes/libarrow_python.pxd

Lines 264 to 269 in ef00568

	cdef extern from "arrow/python/common.h" namespace "arrow::py" nogil:
	cdef cppclass SharedPtrNoGIL[T](shared_ptr[T]):
	# This looks like the only way to satisfy both Cython 2 and Cython 3
	SharedPtrNoGIL& operator=(...)
	cdef cppclass UniquePtrNoGIL[T, DELETER=*](unique_ptr[T, DELETER]):
	UniquePtrNoGIL& operator=(...)

and

arrow/python/pyarrow/lib.pxd

Lines 28 to 34 in ef00568

	# Will be available in Cython 3, not backported
	# ref: https://github.com/cython/cython/issues/3293#issuecomment-1223058101
	cdef extern from "<optional>" namespace "std" nogil:
	cdef cppclass nullopt_t:
	nullopt_t()
	cdef nullopt_t nullopt

This is not strictly required for bumping the required version so it can be tackled as a separate issue.

Component(s)

Python