provision: Replace virtualenv with python native venv.

- Replace virtualenv with python 3's native venv feature. The venv used is native to
  python3.5+, so there's no need for a separate dependency.
- Remove redundant activation script. An activation script is required
  to use the pip and python in the virtual environment, but because we're
  calling the pip inside the venv, we don't need one.

Fixes #625.

tools/provision

@@ -6,8 +6,6 @@ import argparse
 import subprocess
 import glob
 
-from importlib import import_module
-
 CURRENT_DIR = os.path.dirname(os.path.abspath(__file__))
 ZULIP_BOTS_DIR = os.path.join(CURRENT_DIR, '..', 'zulip_bots')
 sys.path.append(ZULIP_BOTS_DIR)
@@ -48,16 +46,19 @@ the Python version this command is executed with."""
     venv_dir = os.path.join(base_dir, venv_name)
     if not os.path.isdir(venv_dir):
         try:
-            return_code = subprocess.call(['virtualenv', '-p', options.python_interpreter, venv_dir])
+            return_code = subprocess.call([options.python_interpreter, '-m', 'venv', venv_dir])
         except OSError:
-            if subprocess.call(['which', 'virtualenv']):
+            print("{red}Installation with venv failed. Probable errors are: "
-                print("{red}Please install the virtualenv package and try again.{end_format}"
+                  "You are on Ubuntu and you haven't installed python3-venv,"
-                      .format(red='\033[91m', end_format='\033[0m'))
+                  "or you are running an unsupported python version"
-                sys.exit(1)
+                  "or python is not installed properly{end_format}"
+                  .format(red=red, end_format=end_format))
+            sys.exit(1)
             raise
         else:
+            # subprocess.call returns 0 if a script executed successfully
             if return_code:
-                raise OSError("The command `virtualenv -p {} {}` failed. Virtualenv not created!"
+                raise OSError("The command `{} -m venv {}` failed. Virtualenv not created!"
                               .format(options.python_interpreter, venv_dir))
             print("New virtualenv created.")
     else:
@@ -65,20 +66,12 @@ the Python version this command is executed with."""
 
     if os.path.isdir(os.path.join(venv_dir, 'Scripts')):
         # POSIX compatibility layer and Linux environment emulation for Windows
-        # Virtual uses /Scripts instead of /bin on Windows.
+        # venv uses /Scripts instead of /bin on Windows cmd and Power Shell.
-        # Read https://virtualenv.pypa.io/en/stable/userguide/
+        # Read https://docs.python.org/3/library/venv.html
         venv_exec_dir = 'Scripts'
     else:
         venv_exec_dir = 'bin'
 
-    # In order to install all required packages for the venv, we need to activate it. Since
-    # the activation script sets environmental variables, it needs to be executed inline with
-    # `import_module`.
-    activate_module_dir = os.path.abspath(os.path.join(venv_dir, venv_exec_dir))
-    sys.path.append(activate_module_dir)
-
-    import_module('activate_this')
-
     # On OS X, ensure we use the virtualenv version of the python binary for
     # future subprocesses instead of the version that this script was launched with. See
     # https://stackoverflow.com/questions/26323852/whats-the-meaning-of-pyvenv-launcher-environment-variable