bpo-31985: Deprecate openfp in aifc, sunau, and wave (#4344)

The openfp functions of aifp, sunau, and wave had pointed to the open
function of each module since 1993 as a matter of backwards
compatibility. In the case of aifc.openfp, it was both undocumented
and untested. This change begins the formal deprecation of those
openfp functions, with their removal coming in 3.9.

This additionally adds a TODO in test_pyclbr around using aifc.openfp,
though it shouldn't be changed until removal in 3.9.
diff --git a/Lib/sunau.py b/Lib/sunau.py
index 0473e9e..dbad3db 100644
--- a/Lib/sunau.py
+++ b/Lib/sunau.py
@@ -104,6 +104,7 @@
 """
 
 from collections import namedtuple
+import warnings
 
 _sunau_params = namedtuple('_sunau_params',
                            'nchannels sampwidth framerate nframes comptype compname')
@@ -522,4 +523,7 @@
     else:
         raise Error("mode must be 'r', 'rb', 'w', or 'wb'")
 
-openfp = open
+def openfp(f, mode=None):
+    warnings.warn("sunau.openfp is deprecated since Python 3.7. "
+                  "Use sunau.open instead.", DeprecationWarning, stacklevel=2)
+    return open(f, mode=mode)