python-language-server[all] not working if installing with pip3

ice-python doesn't work when installing with the code `python3 -m pip install python-language-server[all]`. It does with `python -m pip install python-language-server[all]`. I have faced this problem in other packages, could someone explain me why does this happen?

As I have understood, pip is not going to be supported. So I'd rather prefer to install everything with pip3, instead of pip. Although, when I install a package with pip it's also available for python3 kernel.

I wouldn't like to have many duplicates, that's why I try to install all the packages with pip3. If you install with pip is the same than pip3? Problems like the one described above make me nuts to install packages with pip instead pip3.

まず、`python3 -m pip install python-language-server[all]` でインストールを再現し、`python -m pip install python-language-server[all]` の場合と比較します。使用される Python 環境と kernel 環境も確認してください。問題がこのプロジェクトに該当するかどうかを判断し、インストールの動作とサポートされる pip コマンドを文書化または修正できれば完了です。

索引モデルが issue の本文から書いたものです。