Python 3.7+: Add testserver/test/__init__.py

[pzrq]

Note, this is already pointing to the dynamic pystone, as per PR #638.

As discussed in person, there are Good Reasons to not rename/move this, and documentation of pyperf/pyperformance can follow in a future PR.

This resolves an ImportError on brew-installed Python 3.7.3 on macOS 10.14.6:

$ python manage.py runserver
Watching for file changes with StatReloader
Performing system checks...

Exception in thread django-main-thread:
Traceback (most recent call last):
File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/threading.py", line 917, in _bootstrap_inner
self.run()
File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/threading.py", line 865, in run
self._target(*self._args, **self._kwargs)
File "/Users/pzrq/Projects/beeware/venv/lib/python3.7/site-packages/django/utils/autoreload.py", line 54, in wrapper
fn(*args, **kwargs)
File "/Users/pzrq/Projects/beeware/venv/lib/python3.7/site-packages/django/core/management/commands/runserver.py", line 117, in inner_run
self.check(display_num_errors=True)
File "/Users/pzrq/Projects/beeware/venv/lib/python3.7/site-packages/django/core/management/base.py", line 390, in check
include_deployment_checks=include_deployment_checks,
File "/Users/pzrq/Projects/beeware/venv/lib/python3.7/site-packages/django/core/management/base.py", line 377, in _run_checks
return checks.run_checks(**kwargs)
File "/Users/pzrq/Projects/beeware/venv/lib/python3.7/site-packages/django/core/checks/registry.py", line 72, in run_checks
new_errors = check(app_configs=app_configs)
File "/Users/pzrq/Projects/beeware/venv/lib/python3.7/site-packages/django/core/checks/urls.py", line 40, in check_url_namespaces_unique
all_namespaces = _load_all_namespaces(resolver)
File "/Users/pzrq/Projects/beeware/venv/lib/python3.7/site-packages/django/core/checks/urls.py", line 57, in _load_all_namespaces
url_patterns = getattr(resolver, 'url_patterns', [])
File "/Users/pzrq/Projects/beeware/venv/lib/python3.7/site-packages/django/utils/functional.py", line 80, in __get__
res = instance.__dict__[self.name] = self.func(instance)
File "/Users/pzrq/Projects/beeware/venv/lib/python3.7/site-packages/django/urls/resolvers.py", line 579, in url_patterns
patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
File "/Users/pzrq/Projects/beeware/venv/lib/python3.7/site-packages/django/utils/functional.py", line 80, in __get__
res = instance.__dict__[self.name] = self.func(instance)
File "/Users/pzrq/Projects/beeware/venv/lib/python3.7/site-packages/django/urls/resolvers.py", line 572, in urlconf_module
return import_module(self.urlconf_name)
File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/importlib/__init__.py", line 127, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
File "<frozen importlib._bootstrap>", line 983, in _find_and_load
File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 728, in exec_module
File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
File "/Users/pzrq/Projects/beeware/batavia/testserver/urls.py", line 8, in <module>
from test import pystone
ImportError: cannot import name 'pystone' from 'test' (/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/test/__init__.py)

See also issue #731, PR #741.

PR Checklist:

• [x] All new features have been tested
• [x] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct