Add support for the callback in python/cffi

This uses one global event handler function on the C level, as
enforced by CFFI, and caters for registering a per-context event
handeler.  A simple fixture for doing this during tests is also
implemented.

python/tests/conftest.py

@@ -0,0 +1,15 @@
+import pytest
+
+import deltachat
+
+
+@pytest.fixture
+def register_dc_callback(monkeypatch):
+    """Register a callback for a given context.
+
+    This is a function-scoped fixture and the function will be
+    unregisterd automatically on fixture teardown.
+    """
+    def register_dc_callback(ctx, func):
+        monkeypatch.setitem(deltachat._DC_CALLBACK_MAP, ctx, func)
+    return register_dc_callback

python/tests/test_mrmailbox.py

@@ -1,10 +1,17 @@
-import pathlib
-
-import pytest
-
+import deltachat
 from deltachat import capi
 
 
 def test_empty_context():
     ctx = capi.lib.dc_context_new(capi.ffi.NULL, capi.ffi.NULL, capi.ffi.NULL)
     capi.lib.dc_close(ctx)
+
+
+def test_cb(register_dc_callback):
+    def cb(ctx, evt, data1, data2):
+        return 0
+    ctx = capi.lib.dc_context_new(capi.lib.py_dc_callback,
+                                  capi.ffi.NULL, capi.ffi.NULL)
+    register_dc_callback(ctx, cb)
+    capi.lib.dc_close(ctx)
+    assert deltachat._DC_CALLBACK_MAP[ctx] is cb