Fix sys.stdin.seek() problem properly.

The last hotfix to make sys.stdin seek()-able was far from being
proper. It checked only for sys.stdin (which is only one possible
stream delivering unseekable file positions) and didn't consider that
sys.stdin could be replaced (by somthing with proper seek
functionality).

We now test properly for working seek() functionality. If the stream
does not support it, we copy streamdata to a new temporary file. If
the file is not too large, we keep the file data in memory (up to 20
MB), so in most cases runtime trade-off should be moderate.

diceware/wordlist.py

@@ -17,11 +17,7 @@
 """
 import os
 import re
-import sys
-try:                                 # pragma: no cover
-    from StringIO import StringIO
-except ImportError:                  # pragma: no cover
-    from io import StringIO
+import tempfile
 
 #: The directory in which wordlists are stored
 WORDLISTS_DIR = os.path.abspath(
@@ -94,14 +90,21 @@ class WordList(object):
     """
     def __init__(self, path_or_filelike=None):
         self.path = None
-        if path_or_filelike is sys.stdin:
-            path_or_filelike = StringIO(path_or_filelike.read())
         if not hasattr(path_or_filelike, 'read'):
             # got a path, not a filelike object
             self.path = path_or_filelike
             self.fd = open(self.path, "r")
         else:
             self.fd = path_or_filelike
+            try:
+                self.fd.seek(0)
+            except IOError:
+                # the given filelike does not support seek(). Create an own.
+                max_size = 20 * 1024 * 1024  # 20 MB
+                self.fd = tempfile.SpooledTemporaryFile(
+                    max_size=max_size, mode="w+")
+                self.fd.write(path_or_filelike.read())
+                self.fd.seek(0)
         self.signed = self.is_signed()
 
     def __iter__(self):