bonfire: use pytest_3

Otherwise, tests fail with
> Failed: [pytest] section in setup.cfg files is no longer supported, change to [tool:pytest] instead.

pkgs/tools/misc/bonfire/default.nix

@@ -30,7 +30,7 @@ buildPythonApplication rec {
       --replace "data_files = *.rst, *.txt" ""
   '';
 
-  buildInputs = [ httpretty pytest pytestcov ];
+  buildInputs = [ httpretty pytest_3 pytestcov ];
 
   propagatedBuildInputs = [ arrow click keyring parsedatetime requests six termcolor ];