cmd_send/receive: work around python3.7.0 bug

See https://bugs.python.org/issue26175 . tempfile.SpooledTemporaryFile
doesn't fully implement the IOBase abstract class, which breaks because
py3.7.0's new zipfile module tries to delegate .seekable down to the wrapped
file and causes an AttributeError.

refs #306

src/wormhole/cli/cmd_receive.py

@@ -331,7 +331,14 @@ class Receiver:
         self._msg(u"%d files, %s (uncompressed)" %
                   (file_data["numfiles"], naturalsize(file_data["numbytes"])))
         self._ask_permission()
-        return tempfile.SpooledTemporaryFile()
+        f = tempfile.SpooledTemporaryFile()
+        # workaround for https://bugs.python.org/issue26175 (STF doesn't
+        # fully implement IOBase abstract class), which breaks the new
+        # zipfile in py3.7.0 that expects .seekable
+        if not hasattr(f, "seekable"):
+            # AFAICT all the filetypes that STF wraps can seek
+            f.seekable = lambda: True
+        return f
 
     def _decide_destname(self, mode, destname):
         # the basename() is intended to protect us against

src/wormhole/cli/cmd_send.py

@@ -314,6 +314,12 @@ class Sender:
             # We're sending a directory. Create a zipfile in a tempdir and
             # send that.
             fd_to_send = tempfile.SpooledTemporaryFile()
+            # workaround for https://bugs.python.org/issue26175 (STF doesn't
+            # fully implement IOBase abstract class), which breaks the new
+            # zipfile in py3.7.0 that expects .seekable
+            if not hasattr(fd_to_send, "seekable"):
+                # AFAICT all the filetypes that STF wraps can seek
+                fd_to_send.seekable = lambda: True
             num_files = 0
             num_bytes = 0
             tostrip = len(what.split(os.sep))