Ensure in_testing_environment and matplottlib are resolved earlier in…

… doctest

doc/OnlineDocs/conf.py

@@ -281,6 +281,10 @@ def check_output(self, want, got, optionflags):
     platform.python_implementation()
 )
 
+# Mark that we are testing code (in this case, testing the documentation)
+from pyomo.common.flags import in_testing_environment
+in_testing_environment(True)
+
 # We need multiprocessing because some doctests must be skipped if the
 # start method is not "fork"
 import multiprocessing
@@ -296,6 +300,9 @@ def check_output(self, want, got, optionflags):
 )
 from pyomo.contrib.parmest.parmest import parmest_available
 
+# Ensure that the matplotlib import has been resolved (and the backend changed)
+bool(matplotlib_available)
+
 # Not using SolverFactory to check solver availability because
 # as of June 2020 there is no way to suppress warnings when 
 # solvers are not available
@@ -326,10 +333,6 @@ def check_output(self, want, got, optionflags):
     ma27_available = False
     mumps_available = False
 
-# Mark that we are testing code (in this case, testing the documentation)
-from pyomo.common.flags import in_testing_environment
-in_testing_environment(True)
-
 # Prevent any Pyomo logs from propagating up to the doctest logger
 import logging
 logging.getLogger('pyomo').propagate = False