[FIX] core: WatchedFileHandler compat for 3.7

Fine tuning of 8eaac97, the `errors` attribute was added in py38[^1]

[^1]: python/cpython@ca7b504a4d

closes odoo/odoo#112588

X-original-commit: c6c19ff6a3093fe64035d9f970ab35522f8080f5
Signed-off-by: Julien Castiaux (juc) <juc@odoo.com>

odoo/netsvc.py

@@ -28,6 +28,7 @@ def log(logger, level, prefix, msg, depth=None):
 
 class WatchedFileHandler(logging.handlers.WatchedFileHandler):
     def __init__(self, filename):
+        self.errors = None  # py38
         super().__init__(filename)
         # Unfix bpo-26789, in case the fix is present
         self._builtin_open = None