feat: create a pytest plugin

[guotin]

Created the plugin as in pytest docs

• Two cli flags for pytest, --rts and --rts-init. Maybe the --rts-init is pointless?
• Copied the logic from select.py
• PYTEST_DONT_REWRITE prevented the current implementation of collecting "newly added tests" from throwing an error
• Noticed an error of file being left open in def line_mapping() and tried to fix it
• Changed the tests to use the plugin
  • Would using pytester to test the plugin provide any benefits? (as in pytest plugin docs)
• I guess quite a bit of refactoring could be done now like removing the old scripts and useless files

[matveypashkovskiy]

Would using pytester to test the plugin provide any benefits? (as in pytest plugin docs)

Have no experience with it, maybe @alexey-vyskubov can answer. But in general, if you feel it is beneficial go ahead and use it.

I guess quite a bit of refactoring could be done now like removing the old scripts and useless files

Would be nice

[matveypashkovskiy]

BTW @guotin could you please updated docs as well?