Update setup.py to accomodate pip 20+

[Brandl]

This fixes a problem with the newest version of pip:

Traceback (most recent call last):
  File "scrapy-selenium/setup.py", line 5, in <module>
    from pip.download import PipSession
ModuleNotFoundError: No module named 'pip.download'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "scrapy-selenium/setup.py", line 10, in <module>
    from pip._internal.download import PipSession
ModuleNotFoundError: No module named 'pip._internal.download'

[tristanlatr]

@clemfromspace

This pip issue is causing major install problems on all softwares using scrapy-selenium as a pip requirement.
Can you merge this patch or change setup.py to use static install_requires field (as it should be the case actually...) ?

Merci beaucoup !