Fix compatibility with click version 8

[frenzymadness]

In the new version of click, if an option has no type and a default value is of type bool, the default type is bool and the required value for the option is also bool which is not correct for options like --code, --requirements, or --python.

Click 7:

# pipenv install --help | egrep "TEXT|BOOLEAN"
  -c, --code TEXT                 Install packages automatically discovered
  -e, --editable TEXT             An editable Python package URL or path,
  -r, --requirements TEXT         Import a requirements.txt file.
  --extra-index-url TEXT          URLs to the extra PyPI compatible indexes to
  -i, --index TEXT                Target PyPI-compatible package index url.
  --python TEXT                   Specify which version of Python virtualenv
  --pypi-mirror TEXT              Specify a PyPI mirror.

Click 8:

# pipenv install --help | egrep "TEXT|BOOLEAN"
  -c, --code BOOLEAN              Install packages automatically discovered
  -e, --editable TEXT             An editable Python package URL or path,
  -r, --requirements BOOLEAN      Import a requirements.txt file.
  --extra-index-url TEXT          URLs to the extra PyPI compatible indexes to
  -i, --index TEXT                Target PyPI-compatible package index url.
  --python BOOLEAN                Specify which version of Python virtualenv
  --pypi-mirror TEXT              Specify a PyPI mirror.

As you can see above, the new behavior is not a problem for some of the options because --index and --editable have a specific type, --extra-index-url has a string as a default value, etc.

This change should not affect pipenv in any way and should make it more compatible with the new version of click which pipenv surely vendor soon.

Also, explicit is better than implicit 😃

[frenzymadness]

I think that the failure of the CI is not related to this change. It seems that the vulnerability check in the tests ignores vulnerability with id 350156 but there is another one with id 40291 in pip <21.1 which causes the check to end with return code 1.

There is an open PR with WIP update of pip to the latest version #4606

[frenzymadness]

Updated. It seems to be safer to have the default set as an empty string. Otherwise False might be transferred to string and then used which might cause a problem because /usr/bin/false exists for example for --python option.

[frenzymadness]

This PR is not complete at all. I'm afraid that there are many more issues than I thought. From what I see now:

• click 8 does not support Python 2 anymore, pipenv seems to still support it
• there are bundled plugins like click-completion which has unclear future and are not compatible with the latest click so it's not possible to update them: Clarify whether this repo is deprecated or not click-contrib/click-completion#41