Pass environment to subprocess

- `check_output` needs to pass `PYTHONIOENCODING` to `Popen` on python2

Signed-off-by: Dan Ryan <dan@danryan.co>

tests/unit/test_help.py

@@ -1,3 +1,4 @@
+import os
 import subprocess
 import sys
 
@@ -5,6 +6,6 @@ import sys
 def test_help():
     output = subprocess.check_output(
         [sys.executable, '-m', 'pipenv.help'],
-        stderr=subprocess.STDOUT,
+        stderr=subprocess.STDOUT, env=os.environ.copy(),
     )
     assert output