Restore support for Python 2.7.0-2.7.8

`hashlib.algorithms_guaranteed` was introduced only in Python 2.7.9, see https://docs.python.org/2.7/library/hashlib.html#hashlib.algorithms_guaranteed

Ubuntu 14.04 LTS (which will be supported until April 2019) still provides python 2.7.5 and therefore needs this fix.

Tested on python 2.7.3, 2.7.5, 2.7.12 and 3.5.3.

Author: nsoranzo

bagit.py

@@ -122,7 +122,11 @@ def find_locale_dir():
     # Payload-Oxum is autogenerated
 ]
 
-CHECKSUM_ALGOS = hashlib.algorithms_guaranteed
+try:
+    CHECKSUM_ALGOS = hashlib.algorithms_guaranteed
+except AttributeError:
+    # Python 2.7.0-2.7.8
+    CHECKSUM_ALGOS = set(hashlib.algorithms)
 DEFAULT_CHECKSUMS = ["sha256", "sha512"]
 
 #: Block size used when reading files for hashing: