common/reloader: import reload from importlib (#2093)

The imp module is deprecated since Python 3.4 [1]. The import was added
even after it was deprecated in [2]. In Python 3.12 the imp module will
be removed.

Use the reload function from importlib, which is the designated
replacement [1].

With #2091 and this change, crossbar can be installed and run on
Python 3.12 with:
- numpy>=1.26.0b1
- wsaccel @ git+https://github.com/methane/[email protected] (assuming
  cython>=3.0.0 is installed, tarball from pypi won't work [3])

[1] https://docs.python.org/3.11/library/imp.html
[2] 21910b0
[3] methane/wsaccel#30

crossbar/common/reloader.py

@@ -14,7 +14,7 @@
     reload
 except NameError:
     # Python 3
-    from imp import reload
+    from importlib import reload
 
 __all__ = ('TrackingModuleReloader', )