Handle wait_gen_kwargs which take an argument

All wait_gen args are subject to maybe_call() as a way to dynamically
look up a value at runtime.  These callables do not take an argument.
If a callable which takes an argument is passed (as is the case with
the `value` kwargs for the `runtime` wait generator, we pass through
the function.

Author: Bob Green

backoff/_common.py

@@ -15,7 +15,13 @@
 
 # Evaluate arg that can be either a fixed value or a callable.
 def _maybe_call(f, *args, **kwargs):
-    return f(*args, **kwargs) if callable(f) else f
+    if callable(f):
+        try:
+            return f(*args, **kwargs)
+        except TypeError:
+            return f
+    else:
+        return f
 
 
 def _init_wait_gen(wait_gen, wait_gen_kwargs):