Add set_python_envvars.sh.

tests/letstest/scripts/set_python_envvars.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+# This is a simple script that can be sourced to set Python environment
+# variables for use in Certbot's letstest test farm tests.
+
+# Some distros like Fedora may only have an executable named python3 installed.
+if command -v python; then
+    PYTHON_NAME="python"
+    VENV_SCRIPT="tools/venv.py"
+    VENV_PATH="venv"
+else
+    # We could check for "python2" here, however, the addition of "python3"
+    # only systems is what necessitated this change so checking for "python2"
+    # isn't necessary.
+    PYTHON_NAME="python3"
+    VENV_PATH="venv3"
+    VENV_SCRIPT="tools/venv3.py"
+fi