Prevent a traceback when comparing a contentful .pyc file with an empty one. (Re: Debian:#1064973)

They were being detected as PycFiles via FALLBACK_FILE_EXTENSION_SUFFIX,
but this allowed (valid, it seems!) empty .pyc files to be processed by
code that assumed that the file was non-empty. This patch continues
to detect them as PycFiles but does not attempt in-depth parsing.

diffoscope/comparators/python.py

@@ -21,6 +21,7 @@ import binascii
 import dis
 import io
 import marshal
+import os
 import re
 import struct
 import time
@@ -66,6 +67,12 @@ def describe_pyc(filename):
 
 
 def parse_pyc(f):
+    f.seek(0, io.SEEK_END)
+    if f.tell() == 0:
+        yield "type:     empty"
+        return
+
+    f.seek(0)
     magic = f.read(4)
     yield "magic:    {}".format(hexlify(magic))