Switch to python3 in bindep.txt

We've already dropped py2 support and switched to py3. This was missed.

Change-Id: I8484ffc8993b28fca25885d3f712ac5c0a8939b8
Partially-Implements: blueprint drop-py2-support

bindep.txt

@@ -3,7 +3,7 @@ libffi-dev [platform:dpkg]
 libffi-devel [platform:rpm]
 gcc [platform:rpm]
 gcc [platform:dpkg]
-python-dev [platform:dpkg]
-python-devel [platform:rpm]
+python3-dev [platform:dpkg]
+python3-devel [platform:rpm]
 openssl-devel [platform:rpm]
 libssl-dev [platform:dpkg]