Move WECHATY_PUPPET_SERVICE_TOKEN checking code in Python Wechaty internally

[huan]

python-wechaty-getting-started/examples/ding-dong-bot.py

Line 70 in 85d082c

if 'WECHATY_PUPPET_SERVICE_TOKEN' not in os.environ:

From the above code, we can see that we are checking the WECHATY_PUPPET_SERVICE_TOKEN in our end user's code.

I'd like to move this checking code into the Python Wechaty module inside, which will make everything clear:

1. the user can provide a token as options to Wechaty, this will override any environment variables that already set.
2. If the user did not provide token as options to Wechaty, then the module will use the environment variable WECHATY_PUPPET_SERVICE_TOKEN

See also: TS Wechaty Puppet Service source code:

1. https://github.com/wechaty/wechaty-puppet-service/blob/5d7e83db2beba891ce9ee71be69542739064fc52/src/client/puppet-service.ts#L159
2. https://github.com/wechaty/wechaty-puppet-service/blob/5d7e83db2beba891ce9ee71be69542739064fc52/src/config.ts#L13-L32

[wj-Mcat]

This is a third-warning for beginner to set WECHATY_PUPPET_SERVICE_TOKEN envrionment variable.

There are many problems for beginer developers. But the most common problem is How to set token for python-wechaty.

• There are many docs for Token settings method, eg: wechaty.js.org, python-wechaty. But there are also many beginner who don't know how to set token.
• This code is in hard-code that is abvious that token must be set.

This is my consideration.