Add tutorial doctest module and enable top level await (#919)

Add a tutorial module with examples that can be tested with `doctest`.

In order to simplify the examples they can be run with doctest allowing
top level await statements by adding a fixture to patch the builtins
that xdoctest uses to test code.

---------

Co-authored-by: Teemu R. <tpr@iki.fi>

docs/source/tutorial.md

@@ -0,0 +1,8 @@
+# Tutorial
+
+```{eval-rst}
+.. automodule:: tutorial
+    :members:
+    :inherited-members:
+    :undoc-members:
+```