Refactor the package

[vava24680]

In this PR, I have done the following changes:

• Stop supporting Python 2 since it has reached its EOL
• Remove the third-party argparse dependency and use the one provided by the Python 3 Standard Library
• Add flake8 as a linter
• Add the new command line interface and the console script, the screenshot is shown as below
• Remove the sub-command --all since this functionality does not show up in the Chinese version data sheet nor the English version data sheet, so I removed it to prevent the misusing
• Use a class RPi_MH_Z19B to encapsulate all the functions, in this way, it can also be used a dependency in another project or package
• Make every command sequence as a variable in the const module, which makes it more readable.
• Remove the start_getty and the stop_getty functions since I think both of them is unnecessary if the TX and the RX pins are connected a MH-Z19B sensor, the getty should not work on those two pins

[UedaTakeyuki]

Thank you for your great works on this repository. I can see these works are fine, modern, good professional manner works. I'm moving and filling to prise your great works.

Unfortunately, there is a lot of interface breaking change, this is the cause of this **rejection of your pull request". There are already many users of this library in the world. I really appreciate these users and respect their situation to use.

For example, all() feature is requested by one of these users, so we should respect this request, keep it to remain. Python2 interface is also, I can't find any reasonable reason to abandon an environment that is using python2 still.

I hope you get to use this library and to like also. In case there are some pieces of stuff to block your using, the change to fix it must be reasonable to accept. To say frank, I can't find any change which needs some of these, on the contrary, all of your change seems to just refactoring which helps no new user who is stacking to use this now. I hope you tell me your reason to block your using and really welcome your pull request just fixing your stacking issue to use this without any refactoring. Thank you for your respect for the community.