Add guppy backend

[dxe4]

related to my comment here https://github.com/pythonprofilers/memory_profiler/issues/236#issuecomment-549044946 with implementation of guppy3 we can get the right values.

note this implementation does self.prevlines[-1] == self.prev_lineno: for guppy backend only, because repeating [i for i in range(0, 10000000)] makes the guppy backend extremely slow. although this may not be ideal or acceptable i thought it's worth sharing the code.

@profile
def myfn_py():
    ones = [1] * NBR_ITEMS
    twos = [i + 1 for i in ones]
    return twos

@profile
def myfn_np():
    ones_np = np.ones(NBR_ITEMS)
    twos_np = ones_np+1
    return twos_np

myfn_np()
myfn_py()

python -m memory_profiler --backend guppy prof.py 

Line #    Mem usage    Increment   Line Contents
================================================
    11   14.191 MiB   14.191 MiB   @profile
    12                             def myfn_py():
    13   18.006 MiB    3.815 MiB       ones = [1] * NBR_ITEMS
    14   22.098 MiB    4.091 MiB       twos = [i + 1 for i in ones]
    15                                 return twos

Filename: prof.py

Line #    Mem usage    Increment   Line Contents
================================================
    18   14.189 MiB   14.189 MiB   @profile
    19                             def myfn_np():
    20   18.005 MiB    3.817 MiB       ones_np = np.ones(NBR_ITEMS)
    21   21.820 MiB    3.815 MiB       twos_np = ones_np+1
    22   21.820 MiB    0.000 MiB       return twos_np