Introduce pytest command as recommended entry point

Fixes #1629

doc/en/example/parametrize.rst

@@ -44,14 +44,14 @@ Now we add a test configuration like this::
 
 This means that we only run 2 tests if we do not pass ``--all``::
 
-    $ py.test -q test_compute.py
+    $ pytest -q test_compute.py
     ..
     2 passed in 0.12 seconds
 
 We run only two computations, so we see two dots.
 let's run the full monty::
 
-    $ py.test -q --all
+    $ pytest -q --all
     ....F
     ======= FAILURES ========
     _______ test_compute[4] ________
@@ -128,7 +128,7 @@ label generated by ``idfn``, but because we didn't generate a label for ``timede
 objects, they are still using the default pytest representation::
 
 
-    $ py.test test_time.py --collect-only
+    $ pytest test_time.py --collect-only
     ======= test session starts ========
     platform linux -- Python 3.5.1, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
     rootdir: $REGENDOC_TMPDIR, inifile: 
@@ -179,7 +179,7 @@ only have to work a bit to construct the correct arguments for pytest's
 
 this is a fully self-contained example which you can run with::
 
-    $ py.test test_scenarios.py
+    $ pytest test_scenarios.py
     ======= test session starts ========
     platform linux -- Python 3.5.1, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
     rootdir: $REGENDOC_TMPDIR, inifile: 
@@ -192,7 +192,7 @@ this is a fully self-contained example which you can run with::
 If you just collect tests you'll also nicely see 'advanced' and 'basic' as variants for the test function::
 
 
-    $ py.test --collect-only test_scenarios.py
+    $ pytest --collect-only test_scenarios.py
     ======= test session starts ========
     platform linux -- Python 3.5.1, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
     rootdir: $REGENDOC_TMPDIR, inifile: 
@@ -257,7 +257,7 @@ creates a database object for the actual test invocations::
 
 Let's first see how it looks like at collection time::
 
-    $ py.test test_backends.py --collect-only
+    $ pytest test_backends.py --collect-only
     ======= test session starts ========
     platform linux -- Python 3.5.1, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
     rootdir: $REGENDOC_TMPDIR, inifile: 
@@ -270,7 +270,7 @@ Let's first see how it looks like at collection time::
 
 And then when we run the test::
 
-    $ py.test -q test_backends.py
+    $ pytest -q test_backends.py
     .F
     ======= FAILURES ========
     _______ test_db_initialized[d2] ________
@@ -318,7 +318,7 @@ will be passed to respective fixture function::
 
 The result of this test will be successful::
 
-    $ py.test test_indirect_list.py --collect-only
+    $ pytest test_indirect_list.py --collect-only
     ======= test session starts ========
     platform linux -- Python 3.5.1, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
     rootdir: $REGENDOC_TMPDIR, inifile: 
@@ -366,7 +366,7 @@ parametrizer`_ but in a lot less code::
 Our test generator looks up a class-level definition which specifies which
 argument sets to use for each test function.  Let's run it::
 
-    $ py.test -q
+    $ pytest -q
     F..
     ======= FAILURES ========
     _______ TestClass.test_equals[1-2] ________
@@ -396,7 +396,7 @@ is to be run with different sets of arguments for its three arguments:
 
 Running it results in some skips if we don't have all the python interpreters installed and otherwise runs all combinations (5 interpreters times 5 interpreters times 3 objects to serialize/deserialize)::
 
-   . $ py.test -rs -q multipython.py
+   . $ pytest -rs -q multipython.py
    ...........................
    27 passed in 0.12 seconds
 
@@ -443,7 +443,7 @@ And finally a little test module::
 
 If you run this with reporting for skips enabled::
 
-    $ py.test -rs test_module.py
+    $ pytest -rs test_module.py
     ======= test session starts ========
     platform linux -- Python 3.5.1, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
     rootdir: $REGENDOC_TMPDIR, inifile: