Remove python 3.5 references, tests

[itsayellow]

• I have added an entry to docs/changelog.md

Summary of changes

Python 3.5 is End of Life. This PR changes the help text to make the minimum python for venvs now Python 3.6. It also removes the custom code in the tests to test install of a pipx package using python 3.5. (It was likely that that code never ran in CI anyway, because it required the availability of a python3.5 binary.)

Test plan

Tested by running

pipx install --help

This PR:

> pipx install --help       
usage: pipx install [-h] [--include-deps] [--verbose] [--force]
                    [--suffix SUFFIX] [--python PYTHON]
                    [--system-site-packages] [--index-url INDEX_URL]
                    [--editable] [--pip-args PIP_ARGS]
                    package_spec

The install command is the preferred way to globally install apps
from python packages on your system. It creates an isolated virtual
environment for the package, then ensures the package's apps are
accessible on your $PATH.

The result: apps you can run from anywhere, located in packages
you can cleanly upgrade or uninstall. Guaranteed to not have
dependency version conflicts or interfere with your OS's python
packages. 'sudo' is not required to do this.

pipx install PACKAGE_NAME
pipx install --python PYTHON PACKAGE_NAME
pipx install VCS_URL
pipx install ./LOCAL_PATH
pipx install ZIP_FILE
pipx install TAR_GZ_FILE

The PACKAGE_SPEC argument is passed directly to `pip install`.

The default virtual environment location is /Users/mclapp/.local/pipx
and can be overridden by setting the environment variable `PIPX_HOME`
(Virtual Environments will be installed to `$PIPX_HOME/venvs`).

The default app location is /Users/mclapp/.local/bin and can be
overridden by setting the environment variable `PIPX_BIN_DIR`.

The default python executable used to install a package is
/Users/mclapp/git/pipx/venv39/bin/python3 and can be overridden
by setting the environment variable `PIPX_DEFAULT_PYTHON`.

positional arguments:
  package_spec          package name or pip installation spec

optional arguments:
  -h, --help            show this help message and exit
  --include-deps        Include apps of dependent packages
  --verbose
  --force, -f           Modify existing virtual environment and files in
                        PIPX_BIN_DIR
  --suffix SUFFIX       Optional suffix for virtual environment and executable
                        names. NOTE: The suffix feature is experimental and
                        subject to change.
  --python PYTHON       The Python executable used to create the Virtual
                        Environment and run the associated app/apps. Must be
                        v3.6+.
  --system-site-packages
                        Give the virtual environment access to the system
                        site-packages dir.
  --index-url INDEX_URL, -i INDEX_URL
                        Base URL of Python Package Index
  --editable, -e        Install a project in editable mode
  --pip-args PIP_ARGS   Arbitrary pip arguments to pass directly to pip
                        install/upgrade commands

pipx v0.16.1.0:

> pipx install --help
usage: pipx install [-h] [--include-deps] [--verbose] [--force]
                    [--suffix SUFFIX] [--python PYTHON]
                    [--system-site-packages] [--index-url INDEX_URL]
                    [--editable] [--pip-args PIP_ARGS]
                    package_spec

The install command is the preferred way to globally install apps
from python packages on your system. It creates an isolated virtual
environment for the package, then ensures the package's apps are
accessible on your $PATH.

The result: apps you can run from anywhere, located in packages
you can cleanly upgrade or uninstall. Guaranteed to not have
dependency version conflicts or interfere with your OS's python
packages. 'sudo' is not required to do this.

pipx install PACKAGE_NAME
pipx install --python PYTHON PACKAGE_NAME
pipx install VCS_URL
pipx install ./LOCAL_PATH
pipx install ZIP_FILE
pipx install TAR_GZ_FILE

The PACKAGE_SPEC argument is passed directly to `pip install`.

The default virtual environment location is /Users/mclapp/.local/pipx
and can be overridden by setting the environment variable `PIPX_HOME`
(Virtual Environments will be installed to `$PIPX_HOME/venvs`).

The default app location is /Users/mclapp/.local/bin and can be
overridden by setting the environment variable `PIPX_BIN_DIR`.

The default python executable used to install a package is
/usr/local/opt/python@3.9/bin/python3.9 and can be overridden
by setting the environment variable `PIPX_DEFAULT_PYTHON`.

positional arguments:
  package_spec          package name or pip installation spec

optional arguments:
  -h, --help            show this help message and exit
  --include-deps        Include apps of dependent packages
  --verbose
  --force, -f           Modify existing virtual environment and files in
                        PIPX_BIN_DIR
  --suffix SUFFIX       Optional suffix for virtual environment and executable
                        names. NOTE: The suffix feature is experimental and
                        subject to change.
  --python PYTHON       The Python executable used to create the Virtual
                        Environment and run the associated app/apps. Must be
                        v3.5+.
  --system-site-packages
                        Give the virtual environment access to the system
                        site-packages dir.
  --index-url INDEX_URL, -i INDEX_URL
                        Base URL of Python Package Index
  --editable, -e        Install a project in editable mode
  --pip-args PIP_ARGS   Arbitrary pip arguments to pass directly to pip
                        install/upgrade commands

[cs01]

Nice!