Add exception to value alias on ExceptionInfo

For easier compatibility with unittest pytest-dev/unittest2pytest#36

Co-Authored-By: Bruno Oliveira <nicoddemus@gmail.com>

Author: graingert

changelog/5541.feature.rst

@@ -0,0 +1 @@
+Add '.exception' attribute as an alias for '.value' to facilitate porting tests written using unittest.

src/_pytest/_code/code.py

@@ -422,6 +422,16 @@ def type(self):
     def value(self):
         """the exception value"""
         return self._excinfo[1]
+    
+    @property
+    def exception(self):
+        msg = (
+            "The '.exception' attribute is an alias to facilitate porting "
+            "tests written using unittest.\n"
+            "Prefer '.value' in new code."
+        )  
+        warnings.warn(PytestWarning(msg), stacklevel=2)
+        return self.value        
 
     @property
     def tb(self):