Require cramjam lib to be >= 2.7.0 for python >= 3.6

From version 0.7.0 the library `python-snappy` has been
re-written to use internally uses the `cramjam` library.

This new version of `python-snappy` doesn't work well with versions
of `cramjam` < 2.7.0 as the structure of the code is different
than the one used in the new version.

Also for python < 3.7 new `cramjam` libs ( >= 2.7.0) are not available.

So we pinned `python-snappy` to 0.6.1 for python < 3.7, as this old version
is compatible with the `cramjam` library with versions < 2.7.0
and set to use the newer versions of the `python-snappy` and
`cramjam` libs  for python >= 3.7 .

References: bar-156

Signed-off-by: Giulio Calacoci <[email protected]>

setup.py

@@ -111,7 +111,11 @@
             "grpcio",
             "google-cloud-compute",  # requires minimum python3.7
         ],
-        "snappy": ["python-snappy==0.6.1"],
+        "snappy": [
+            'python-snappy==0.6.1; python_version<"3.7"',
+            'python-snappy; python_version>="3.7"',
+            'cramjam >= 2.7.0; python_version>="3.7"',
+        ],
     },
     platforms=["Linux", "Mac OS X"],
     classifiers=[