num_regression fixture now requires pandas to be installed

[nicoddemus]

It seems that after #35, num_regression now also requires pandas, instead of just numpy as before.

With a virtual environment with just numpy and pytest-regression, this simple test fails:

def test(num_regression):
    num_regression.check([])

        try:
            import numpy as np
        except ModuleNotFoundError:
            raise ModuleNotFoundError(import_error_message("Numpy"))
        try:
            import pandas as pd
        except ModuleNotFoundError:
>           raise ModuleNotFoundError(import_error_message("Pandas"))
E           ModuleNotFoundError: 'Pandas' library is an optional dependency and must be installed explicitly when the fixture 'check' is used

.pr\lib\site-packages\pytest_regressions\num_regression.py:73: ModuleNotFoundError

Would it be possible to remove that requirement? cc @648trindade

[nicoddemus]

Actually looking at the diff in #35, seems it was like this before.

cc @tadeu @carlosnewmar

[renefritze]

How about install extras that add the right dependencies? So pip install pytest-regressions[numpy] would require numpy+pandas for example.

[nicoddemus]

Sounds good!