setup_venv: Link to the correct Python interpreter.

Travis enables different Python versions through virtual environments,
but it seems that there is a little caveat when we try to create Zulip's
virtual environment by referring Travis' virtual environment; Zulip's
virtual environment refers the system Python. We encountered this
behaviour when we tried to run our backend test suite under Python 3.5
in Travis. 'python3 --version' command before activating Zulip's
virtualenv showed 'Python 3.5.3' and after it showed 'Python 3.4.3'.
This happened when we created the virtual environment using
'virtualenv -p python3'.

The solution seems to be to explicitly give the path of the Python
interpreter in the Travis' virtual environment using 'which python3'.

tools/setup/setup_venvs.py

@@ -9,7 +9,7 @@ if ZULIP_PATH not in sys.path:
     sys.path.append(ZULIP_PATH)
 
 from scripts.lib.setup_venv import setup_virtualenv
-from scripts.lib.zulip_tools import run
+from scripts.lib.zulip_tools import run, subprocess_text_output
 
 OLD_VENV_PATH = "/srv/zulip-venv"
 VENV_PATH = "/srv/zulip-py3-venv"
@@ -18,13 +18,17 @@ DEV_REQS_FILE = os.path.join(ZULIP_PATH, "requirements", "dev_lock.txt")
 
 def main(is_travis=False):
     # type: (bool) -> None
+    # Get the correct Python interpreter. If we don't do this and use
+    # `virtualenv -p python3` to create the venv in Travis, the venv
+    # starts referring to the system Python interpreter.
+    python_interpreter = subprocess_text_output(['which', 'python3'])
     if is_travis:
         setup_virtualenv(VENV_PATH, DEV_REQS_FILE, patch_activate_script=True,
-                         virtualenv_args=['-p', 'python3'])
+                         virtualenv_args=['-p', python_interpreter])
     else:
         run(['sudo', 'rm', '-f', OLD_VENV_PATH])
         setup_virtualenv(VENV_PATH, DEV_REQS_FILE, patch_activate_script=True,
-                         virtualenv_args=['-p', 'python3'])
+                         virtualenv_args=['-p', python_interpreter])
 
 if __name__ == "__main__":
     main()