Adds label information to context repr function.
%R is used to get the repr result of a PyObject, in this case this
object is the LabelObject representing the context's label information.
Helps showing more of the context information when printing out a
context object instance.
>>> import fdisk
>>> cxt = fdisk.Context('disk.bin')
>>> cxt
<libfdisk.Context object at 0x7fec52f39330, label=<libfdisk.Label object
at 0x7fec52f39310, name=gpt>, details=True, readonly=False>
7478453359