partition: add partition type getset

Partition type getset enables modifying and getting the type
of a give libfdisk partition.

The type of a partition is defined using PartType, which
can only be instanced via label specific functions
get_parttype_from_string or get_parttype_from_code.

For example, to set the type of a new partitions to 'EFI System':

>>> import fdisk
>>> pa = fdisk.Partition()
>>> pa.type
>>> cxt = fdisk.Context('./disk.bin', readonly=False)
>>> cxt.create_disklabel('gpt')
>>> efitype = cxt.label.get_parttype_from_string("c12a7328-f81f-11d2-ba4b-00a0c93ec93b")
>>> pa.type = efitype

Following the previous example, getting its current partition type:

>>> pa.type
<libfdisk.PartType object at 0x7f2f0a9a12d0, name=EFI System>

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