message: Update the installation document based on Python 3.6+
  * Update the description based on Ubuntu 16.04 with 18.04,
  since Python version bundled with the former is 3.5,
  which is not already supported
  * Remove obsolete descriptions based on Python <= 3.5
diff: @@ -78,9 +78,9 @@ the required dependencies are installed: ::
  sudo apt-get install build-essential libssl-dev libffi-dev python-dev python-pip libsasl2-dev libldap2-dev
  -**Ubuntu 16.04** If you have python3.5 installed alongside with python2.7, as is default on **Ubuntu 16.04 LTS**, run this command also: ::
  +**Ubuntu 18.04** If you have python3.6 installed alongside with python2.7, as is default on **Ubuntu 18.04 LTS**, run this command also: ::
  - sudo apt-get install build-essential libssl-dev libffi-dev python3.5-dev python-pip libsasl2-dev libldap2-dev
  + sudo apt-get install build-essential libssl-dev libffi-dev python3.6-dev python-pip libsasl2-dev libldap2-dev
  otherwise build for ``cryptography`` fails.
  @@ -105,17 +105,18 @@ attempt it, download `get-pip.py <https://bootstrap.pypa.io/get-pip.py>`_, and r
  Python virtualenv
  -----------------
  -It is recommended to install Superset inside a virtualenv. Python 3 already ships virtualenv, for
  -Python 2 you need to install it. If it's packaged for your operating systems install it from there
  -otherwise you can install from pip: ::
  +It is recommended to install Superset inside a virtualenv. Python 3 already ships virtualenv.
  +But if it's not installed in your environment for some reason, you can install it
  +via the package for your operating systems, otherwise you can install from pip: ::
  pip install virtualenv
  You can create and activate a virtualenv by: ::
  - # virtualenv is shipped in Python 3 as pyvenv
  - virtualenv venv
  - . ./venv/bin/activate
  + # virtualenv is shipped in Python 3.6+ as venv instead of pyvenv.
  + # See https://docs.python.org/3.6/library/venv.html
  + python3 -m venv venv
  + . venv/bin/activate
  On windows the syntax for activating it is a bit different: ::
  