Support Selenium WebDriver 4

[KazuCocoa]

Will update here

The problem

example: appium/ruby_lib_core#175

• Remove logic to handle commands for W3C or OSS
• Add commands Appium needs as W3C, but no pure W3C spec
  • example: IME related commands
  • reference: https://github.com/appium/ruby_lib_core/blob/master/lib/appium_lib_core/common/command/w3c.rb
• create session

[KazuCocoa]

Woking branch: https://github.com/appium/python-client/tree/selenium-4

So far, this does not have other breaking changes

[KazuCocoa]

https://pypi.org/project/Appium-Python-Client/2.0.0b1/ is selenium-webdriver 4.0.0.b1 based one. (This repository's tests were passed)
No differences since last 2.0.0.a0 except for selenium webdriver.

[KazuCocoa]

#636