tox.ini: use {envpython} substituion for path to the virtual Python interpreter

tox.ini

@@ -12,11 +12,11 @@ deps =
     pytest
     -rrequirements.txt
 install_command =
-    python -m pip install -v {opts} {packages}
+    {envpython} -m pip install -v {opts} {packages}
 commands =
     # check that we have the expected Python version and architecture
-    python -c "import sys; print(sys.version)"
-    python -c "import struct; print(struct.calcsize('P') * 8)"
+    {envpython} -c "import sys; print(sys.version)"
+    {envpython} -c "import struct; print(struct.calcsize('P') * 8)"
     # run the test suite
     py.test