replace random with secrets

[AdamJCavanaugh]

Closes #5

As far as I could tell, the try/except wasn't doing anything beneficial, so I simplified by removing it.

Regarding the full replacement, it's what's recommended by the random library itself:

The pseudo-random generators of this module should not be used for security purposes. For security or cryptographic uses, see the secrets module.

• All tests pass successfully
• Running from Python returns random passwords

[soslan]

Thank you for this! Let me explore this more since compatibility with Python 2.7 and 3.5 might be broken here, and the package is still regularly downloaded for those versions.
Also, systems without urandom available should be considered. I am thinking of using an --accept-random flag for users without urandom.

[AdamJCavanaugh]

Great point, thanks. I'll have some time to check 2.7 and 3.5 early next week, hopefully and work to make sure it's non-breaking. You're welcome to do updates before as well, of course.