Update SymPy Dependency + Pipeline Cleanup

This MR

• raises the upper limit for SymPy to 1.12.1
• introduces a weekly test run against the latest sympy pre-release
• cleans up the CI configuration a bit

Closes #91 (closed)

src/pystencils/boundaries/boundaryhandling.py

@@ -35,11 +35,11 @@ class FlagInterface:
         >>> dh = create_data_handling((4, 5))
         >>> fi = FlagInterface(dh, 'flag_field', np.uint8)
         >>> assert dh.has_data('flag_field')
-        >>> fi.reserve_next_flag()
+        >>> int(fi.reserve_next_flag())
         2
-        >>> fi.reserve_flag(4)
+        >>> int(fi.reserve_flag(4))
         4
-        >>> fi.reserve_next_flag()
+        >>> int(fi.reserve_next_flag())
         8
     """