using extra requirements rather than dynamic install requirements

[elonzh]

Issue description

I am using pipenv to manage my project requirements. Pipenv does not support manually specify package installation order, see https://github.com/pypa/pipenv/issues/2274

Even if I use requirements.txt, I still need to specify the signer package before the adb package.

Expected result

Using extras_require to declare optional requirements. Like:

setup(
    name="adb",
    ...
    extras_require={
        'M2Crypto': ["M2Crypto>=0.21.1,<=0.26.4"],
        'rsa': ["rsa"],
        'pycryptodome': ["pycryptodome"], 
    }
)

I will make a pull request if this issue is confirmed.