Make android_backup an optional dependency (#476)

Related to #474

miio/extract_tokens.py

@@ -8,7 +8,6 @@
 
 import attr
 import click
-from android_backup import AndroidBackup
 from cryptography.hazmat.backends import default_backend
 from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
 
@@ -190,6 +189,12 @@ def read_yeelight_database(tar):
     devices = []
     reader = BackupDatabaseReader(dump_raw)
     if backup.endswith(".ab"):
+        try:
+            from android_backup import AndroidBackup
+        except ModuleNotFoundError:
+            click.echo("You need to install android_backup to extract "
+                       "tokens from Android backup files.")
+            return
 
         with AndroidBackup(backup, stream=False) as f:
             tar = f.read_data(password)

setup.py

@@ -50,13 +50,16 @@ def readme():
         'cryptography',
         'zeroconf',
         'attrs',
-        'android_backup',
         'pytz',
         'appdirs',
         'tqdm',
         'netifaces',
     ],
 
+    extras_require={
+        'Android backup extraction': 'android_backup',
+    },
+
     entry_points={
         'console_scripts': [
             'mirobo=miio.vacuum_cli:cli',