Make Flask extension compatible with Flask>=2.2.0

Flask 2.2.0 deprecated the context stack in favour of using Python
ContextVar, and introduced a fake stack for some sort of backward
compatibility. However, it is not compatible enough for us as the
existing code would blindly push context replacing any existing
one, which will break application code.

The extension now does the recommended action of storing its data on
flask.g - in this case we store the context that needs to be popped
since _app_ctx_stack is deprecated.

There are no existing tests for this extension but I've tested it
locally to my satisfaction.

Fixes: #24

spinach/contrib/flask_spinach.py

@@ -1,12 +1,12 @@
 import click
-from flask import current_app, _app_ctx_stack
+import flask
+
 import spinach
 from spinach import signals
 from spinach.const import DEFAULT_WORKER_NUMBER, DEFAULT_QUEUE
 
 
 class Spinach:
-
     def __init__(self, app=None):
         self.app = app
         if app is not None:
@@ -38,19 +38,48 @@ def spinach_run_workers(threads, queue, stop_when_queue_empty):
 
         @signals.job_started.connect_via(namespace)
         def job_started(*args, job=None, **kwargs):
-            app.app_context().push()
+            if not flask.has_app_context():
+                ctx = app.app_context()
+                ctx.push()
+                flask.g.spinach_ctx = ctx
+            self.job_started(job)
 
         @signals.job_finished.connect_via(namespace)
         def job_finished(*args, job=None, **kwargs):
-            _app_ctx_stack.pop()
+            self.job_finished(job)
+            try:
+                flask.g.spinach_ctx.pop()
+            except AttributeError:
+                # This means we didn't create the context. Ignore.
+                pass
+
+    @classmethod
+    def job_started(cls, *args, job=None, **kwargs):
+        """Callback for subclasses to receive job_started signals.
+
+        There's no guarantee of ordering for Signal's callbacks,
+        so use this callback instead to make sure the app context
+        was pushed.
+        """
+        pass
+
+    @classmethod
+    def job_finished(cls, *args, job=None, **kwargs):
+        """Callback for subclasses to receive job_finished signals.
+
+        There's no guarantee of ordering for Signal's callbacks,
+        so use this callback instead to make sure the app context
+        was pushed.
+        """
+        pass
 
     @property
     def spin(self):
         if self.app is not None:
             return self.app.extensions['spinach']
 
         try:
-            return current_app.extensions['spinach']
+            return flask.current_app.extensions['spinach']
         except (AttributeError, TypeError, KeyError):
             raise RuntimeError('Spinach extension not initialized. '
                                'Did you forget to call init_app?')