Added some try/except clauses since MIP backends are an optional extra.

setup.cfg

@@ -27,7 +27,10 @@ universal=1
 [extras]
 contrib = 
     matplotlib >= 2.0
+symmip =
+    pyscipopt
 # all should be a complete list of all dependencies of all other extras. How to
 # automate this?
 all =
     matplotlib >= 2.0
+    pyscipopt

symfit/symmip/backend/__init__.py

@@ -1,2 +1,2 @@
 from .gurobi import GurobiBackend
-from .scipopt import SCIPOptBackend
+from .scipopt import SCIPOptBackend

symfit/symmip/backend/gurobi.py

@@ -1,6 +1,12 @@
 from dataclasses import dataclass, field
-
-import gurobipy as gp
+import importlib
+import warnings
+
+try:
+    gp = importlib.import_module('gurobipy')
+except ImportError as e:
+    warnings.warn('Install gurobipy to use this feature.')
+    gp = {}
 
 from sympy.printing.pycode import PythonCodePrinter
 

symfit/symmip/backend/scipopt.py

@@ -1,6 +1,12 @@
 from dataclasses import dataclass, field
-
-import pyscipopt as scip
+import importlib
+import warnings
+
+try:
+    scip = importlib.import_module('pyscipopt')
+except ImportError as e:
+    warnings.warn('Install pyscipopt to use this feature.')
+    scip = {}
 
 from sympy.printing.pycode import PythonCodePrinter