Remove pip as the primary installation method

[ketozhang]

airflowctl is affected by the chicken-and-egg problem of Python version. No surprisingly it has the same burden as other Python environment & project managers (e.g., hatch, pipx, pyenv) :

If we start with pip, the version used in airflowctl built must match the same version as your pip environment:

  $ python -V
  Python X.Y.Z

  $ python -m pip install airflowctl
  $ airflowctl init --python-version=X.Y.Z .
  $ airflowctl build .

Otherwise pyenv is invoked to get the correct python version (forcing users to use a specific version manager).

The above is the ideal instruction if we're going to use pip. With the current instructions, you can run into a lot of issues if you have one of these situation:

• --python-version is not the same as the version used in pip install airflowctl
• --python-version is not specified
• pyenv is not available
• python is a different version than the one installed with pip. For example:
  • In a broken environment where python and pip are related
  • User later on upgraded Python and did not reinstall airflowctl.

Here are my suggestions:

• Installation instructions should remove pip as the primary method.
• Ideally, airflowctl behaviors should not depend on the Python version it is installed to.
• airflowctl build should fail if python_version does not exist in $PATH. Error message should ask users to have the correct Python version.
  • Alternatively, users can override which Python path to build the virtual environment with.
• Provide a Python-agnostic installation method (e.g., brew, apt, yum, winget, choco, etc.)

Alternatively, although a bigger rewrite, switch to using hatch or rye.