Add setuptools extra_requires for easy --key dependency install

[johnthagen]

Currently the README says:

https://github.com/pyinstaller/pyinstaller#requirements-and-tested-platforms

Python:
- 3.5-3.7
- tinyaes 1.0+ (only if using bytecode encryption)

Which was updated in #4652.

setuptools.extra_requires could be used to simplify installing this optional depedency for the user.

This could be done by modifying setup.py similar to:

setup(
    ...
    extras_require={
        "encryption":  ["tinyaes>=1.0.0"],
    }
)

And then users could install / add to requirements.txt:

$ pip install pyinstaller[encryption]

[Legorooj]

Nice idea. I'll add this today.

[johnthagen]

Just curious, do you know when a release will be produced with this new tinyaes support? Looks really nice for users.

[Legorooj]

@johnthagen the plan is the 4.0 milestone. Then we'll release.

[Legorooj]

Added in 15ecf74. Notes to doc and README added as well.