Add a python interface

[adamchainz]

It would be useful if Python applications could do something like:

import treehugger

my_vars = treehugger.decrypt('''
TREEHUGGER_APP: foo
TREEHUGGER_STAGE: bar
BAZ: {encrypted: 1234}
''')
print(my_vars['BAZ'])

This would be especially useful on Lambda, where the built-in encrypted variable support isn't very useful as it doesn't use any encryption context, so all lambdas need access to decrypt all other lambdas' variables, and any other data encrypted with that key.