Docs: Avoid ambiguity by explicitly invoking python3

[hugovk]

python/pythondotorg#1549 details someone's difficulty pip installing Pillow.

Whilst it's more a general Python issue than a Pillow issue, one reply is (python/pythondotorg#1549 (comment)):

Thanks for the report. I think the main issue here is the continuing confusion between Python 2 and Python 3 command names and scripts, something not unique to macOS installations of Python. The confusion here could have been avoided by better documentation of the differences between using the command nanespython, python2, and python3 and similarly pip, pip2, and pip3. Yes, use of virtual environments is one way to avoid the differences, in that an active Python 3 virtual environment provides a python and pip alias to the Python 3 versions. But use of a virtual environment isn't necessary in situations like this; all that is required is ensuring that you are using the instance of pip that is associated with the instance of Python in use. If one is invoking python with python3 then usually there will be a corresponding pip3 (likewise, for example, python3.8 and pip3.8):

pip3 install pillow

But, even better, you can avoid any ambiguity by invoking pip via the python command line:

python3 -m pip install pillow

Unfortunately, our documentation in this area lags somewhat. With the retirement of Python 2, there are some on-going efforts to improve the docs but there is still much to be done.

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We recently updated the docs to recommend using python -m pip over pip (#4214).

Let's also recommend using python3 -m pip over python -m pip.