Include python36-devel in bindep

After https://review.opendev.org/#/c/751112/
py36 tests fail w/
  psutil/_psutil_common.c:9:10: fatal error: Python.h: No such file or directory
  #include <Python.h>
           ^~~~~~~~~~
  compilation terminated.
  error: command 'gcc' failed with exit status 1

Although this change was reverted, it will eventually be
re-added later in the Wallaby cycle so fix the issue now.

Change-Id: Iab14c77f956e86d6690d1ce7cd55dd43a585b5bc

bindep.txt

@@ -0,0 +1,4 @@
+# This file contains runtime (non-python) dependencies
+# More info at: http://docs.openstack.org/infra/bindep/readme.html
+
+python36-devel [platform:rpm]

tox.ini

@@ -124,6 +124,16 @@ paths =./ovn_octavia_provider/hacking
 envdir = {toxworkdir}/shared
 commands = {toxinidir}/tools/generate_config_file_samples.sh
 
+# This environment can be used to quickly validate that all needed system
+# packages required to successfully execute test targets are installed
+[testenv:bindep]
+# Do not install any requirements. We want this to be fast and work even if
+# system dependencies are missing, since it's used to tell you what system
+# dependencies are missing! This also means that bindep must be installed
+# separately, outside of the requirements files.
+deps = bindep
+commands = bindep test
+
 [testenv:lower-constraints]
 deps =
   -c{toxinidir}/lower-constraints.txt