* refactor: remove usage of global variables in abbrevs.py
* chore: add flake8-mutable to prevent mutable defaults
* fix: abbrevs expand test
* refactor: add xonsh session singleton
* refactor: fix circular errors when using xonshSession as singleton
* refactor: remove black magicked builtin attributes
* style: black format tests as well
* refactor: update tests to use xonsh-session singleton
* refactor: update abbrevs to not use builtins
* test: remove DummyCommandsCache and patch orig class
* fix: failing test_command_completers
* test: use monkeypatch to update xession fixture
* fix: failing test_pipelines
* fix: failing test_main
* chore: run test suit as single invocation
* test: fix tests/test_xonsh.xsh
* refactor: remove builtins from docs/conf.py
* fix: mypy error in jobs
* fix: test error from test_main
* test: close xession error in test_command_completers
* chore: use pytest-cov for reporting coverage
this will include subprocess calls, and will increase coverage
* style:
* delete package ptk; rename ptk2 to ptk_shell.; leave ptk2 as alias for ptk_shell.
* SHELL_TYPE "prompt_toolkit" only; remove ptk1 specific behavior.
* Doc updates: eliminate reference to prompt-toolkit < 2.0
* update requirements files ptk>=2; test shell_style="none"
* fix ptk2 stub per code review
* Add ptk2 to list of packages to install.
If user is on Windows and has `FORCE_POSIX_PATHS` set to False, then
trailing backslash will not enter multiline mode as this messed up
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