Change EnvironmentError, IOError to OSError - they are aliases

Since Python 3.3, these are aliases for OSError:
https://docs.python.org/3/whatsnew/3.3.html#pep-3151-reworking-the-os-and-io-exception-hierarchy

doc/en/example/assertion/failure_demo.py

@@ -167,7 +167,7 @@ class TestRaises:
         raises(TypeError, int, s)
 
     def test_raises_doesnt(self):
-        raises(IOError, int, "3")
+        raises(OSError, int, "3")
 
     def test_raise(self):
         raise ValueError("demo error")

doc/en/example/reportingdemo.rst

@@ -406,7 +406,7 @@ Here is a nice run of several failures and how ``pytest`` presents things:
     self = <failure_demo.TestRaises object at 0xdeadbeef>
 
         def test_raises_doesnt(self):
-    >       raises(IOError, int, "3")
+    >       raises(OSError, int, "3")
     E       Failed: DID NOT RAISE <class 'OSError'>
 
     failure_demo.py:170: Failed