utils: is_binary(): drop check for \xff in byte string

Instead rely on checking for only null bytes in the initial chunk of the
file.

And revert decoding confidence setting to fix tests.

Author: radhermit

src/pkgcheck/utils.py

@@ -91,7 +91,7 @@ def is_binary(path, blocksize=1024):
 
         # guess character encoding using chardet
         detected_encoding = chardet.detect(byte_str)
-        if detected_encoding['confidence'] > 0.6:
+        if detected_encoding['confidence'] > 0.8:
             try:
                 byte_str.decode(encoding=detected_encoding['encoding'])
                 decodable = True
@@ -101,6 +101,6 @@ def is_binary(path, blocksize=1024):
     # finally use all the checks to decide binary or text
     if decodable:
         return False
-    if is_likely_binary or b'\x00' in byte_str or b'\xff' in byte_str:
+    if is_likely_binary or b'\x00' in byte_str:
         return True
     return False