Fix bug #422702: Make flag argument to open optional, and document it that way.

Doc/lib/libdbhash.tex

@@ -21,7 +21,7 @@ This module provides an exception and a function:
   \exception{KeyError}.  It is a synonym for \exception{bsddb.error}.
 \end{excdesc}
 
-\begin{funcdesc}{open}{path, flag\optional{, mode}}
+\begin{funcdesc}{open}{path, \optional{, flag\optional{, mode}}}
   Open a \code{db} database and return the database object.  The
   \var{path} argument is the name of the database file.
 

Lib/dbhash.py

@@ -12,5 +12,5 @@ __all__ = ["error","open"]
 
 error = bsddb.error                     # Exported for anydbm
 
-def open(file, flag, mode=0666):
+def open(file, flag = 'r', mode=0666):
     return bsddb.hashopen(file, flag, mode)