[5.0.x] Added missing pycon directives in various docs.

Backport of 718b32c691 from main

docs/ref/forms/api.txt

@@ -148,6 +148,8 @@ if validation has side effects, those side effects will only be triggered once.
 Returns a ``dict`` that maps fields to their original ``ValidationError``
 instances.
 
+.. code-block:: pycon
+
     >>> f.errors.as_data()
     {'sender': [ValidationError(['Enter a valid email address.'])],
     'subject': [ValidationError(['This field is required.'])]}
@@ -170,6 +172,8 @@ messages in ``Form.errors``.
 
 Returns the errors serialized as JSON.
 
+.. code-block:: pycon
+
     >>> f.errors.as_json()
     {"sender": [{"message": "Enter a valid email address.", "code": "invalid"}],
     "subject": [{"message": "This field is required.", "code": "required"}]}
@@ -325,10 +329,14 @@ Checking which form data has changed
 Use the ``has_changed()`` method on your ``Form`` when you need to check if the
 form data has been changed from the initial data.
 
-    >>> data = {'subject': 'hello',
-    ...         'message': 'Hi there',
-    ...         'sender': 'foo@example.com',
-    ...         'cc_myself': True}
+.. code-block:: pycon
+
+    >>> data = {
+    ...     "subject": "hello",
+    ...     "message": "Hi there",
+    ...     "sender": "foo@example.com",
+    ...     "cc_myself": True,
+    ... }
     >>> f = ContactForm(data, initial=data)
     >>> f.has_changed()
     False
@@ -336,6 +344,8 @@ form data has been changed from the initial data.
 When the form is submitted, we reconstruct it and provide the original data
 so that the comparison can be done:
 
+.. code-block:: pycon
+
     >>> f = ContactForm(request.POST, initial=data)
     >>> f.has_changed()
 
@@ -350,9 +360,12 @@ The ``changed_data`` attribute returns a list of the names of the fields whose
 values in the form's bound data (usually ``request.POST``) differ from what was
 provided in :attr:`~Form.initial`. It returns an empty list if no data differs.
 
+.. code-block:: pycon
+
     >>> f = ContactForm(request.POST, initial=data)
     >>> if f.has_changed():
     ...     print("The following fields changed: %s" % ", ".join(f.changed_data))
+    ...
     >>> f.changed_data
     ['subject', 'message']