Python3 as default interpreter for new projects

Use `python3` as default interpreter to build the project when it's imported/created.

Author: humitos

readthedocs/projects/migrations/0028_default_interpreter_py3.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.9.13 on 2018-09-06 07:26
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('projects', '0027_remove_json_with_html_feature'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='project',
+            name='python_interpreter',
+            field=models.CharField(choices=[('python', 'CPython 2.x'), ('python3', 'CPython 3.x')], default='python3', help_text='The Python interpreter used to create the virtual environment.', max_length=20, verbose_name='Python Interpreter'),
+        ),
+    ]

readthedocs/projects/models.py

@@ -206,7 +206,7 @@ class Project(models.Model):
         _('Python Interpreter'),
         max_length=20,
         choices=constants.PYTHON_CHOICES,
-        default='python',
+        default='python3',
         help_text=_('The Python interpreter used to create the virtual '
                     'environment.'))