bwpass: Pass TTY name to pinentry

The curses-based pinentry defaults to using its standard input stream to
display the prompt, but this fails when stdin is a pipe.  It does
provide a command-line argument to specify a specific file name, though.

src/bwpass.py

@@ -65,8 +65,9 @@ class Pinentry:
 
     def getpin(self):
         codec = locale.getpreferredencoding()
+        tty = os.ttyname(sys.stdin.fileno())
         p = subprocess.Popen(
-            ['pinentry'],
+            ['pinentry', '-T', tty],
             stdin=subprocess.PIPE,
             stdout=subprocess.PIPE,
         )