Support CLI completion of environment variable names

[RH-TLagrone]

How would this feature be useful?

Supporting completion of environment variables names would improve the user experience of the CLI command pipx environment --value VARIABLE.

Describe the solution you'd like

Delegate the work to argcomplete by configuring argparse's built-in support for choices with the canonical list of ENVIRONMENT_VARIABLES that this project already defines.

More specifically, refactor this method invocation (which defines the CLI argument pipx environment --value VARIABLE) to include the Python argument choices=ENVIRONMENT_VARIABLES, i.e.:

p.add_argument("--value", "-V", choices=ENVIRONMENT_VARIABLES, metavar="VARIABLE", help="Print the value of the variable.")

Describe alternatives you've considered

I have not considered any alternatives because the add_argument(choices=...) mechanism is both sufficient and satisfactory on account of being already built-in (i.e. readily-available, reliably implemented, and officially recommended). Similar logic applies to re-using ENVIRONMENT_VARIABLES for the sake of DRY code.

However, it may be necessary to define an additional constant (e.g. VIRTUAL_ENVIRONMENT_VARIABLES) to represent the following derived_values that can be queried via pipx environment --value VARIABLE but whose names are not in ENVIRONMENT_VARIABLES:

• PIPX_LOCAL_VENVS
• PIPX_LOG_DIR
• PIPX_STANDALONE_PYTHON_CACHE_DIR
• PIPX_TRASH_DIR
• PIPX_VENV_CACHEDIR

---

Reference:

ENVIRONMENT_VARIABLES values	derived_values keys
PIPX_BIN_DIR	PIPX_BIN_DIR
PIPX_DEFAULT_PYTHON	PIPX_DEFAULT_PYTHON
PIPX_FETCH_MISSING_PYTHON
PIPX_GLOBAL_BIN_DIR
PIPX_GLOBAL_HOME
PIPX_GLOBAL_MAN_DIR
PIPX_HOME	PIPX_HOME
PIPX_HOME_ALLOW_SPACE	PIPX_HOME_ALLOW_SPACE
	PIPX_LOCAL_VENVS
	PIPX_LOG_DIR
PIPX_MAN_DIR	PIPX_MAN_DIR
PIPX_SHARED_LIBS	PIPX_SHARED_LIBS
	PIPX_STANDALONE_PYTHON_CACHEDIR
	PIPX_TRASH_DIR
	PIPX_VENV_CACHEDIR
USE_EMOJI	USE_EMOJI

[gagandeepp]

@RH-TLagrone @dukecat0 Please assign if available