use tools/sha256sum.py since macos image does not have sha256sum

Author: mattip

tools/gfortran_utils.sh

@@ -127,7 +127,7 @@ if [ "$(uname)" == "Darwin" ]; then
 		;;
         *) echo Did not recognize arch-plat $arch-$plat; return 1 ;;
 	esac
-        if [[ "$(sha256sum gfortran-darwin-${arch}-${type}.tar.gz)" != "${GFORTRAN_SHA}  gfortran-darwin-${arch}-${type}.tar.gz" ]]; then
+        if [[ "$(python3 tools/sha256sum.py gfortran-darwin-${arch}-${type}.tar.gz)" != "${GFORTRAN_SHA}  gfortran-darwin-${arch}-${type}.tar.gz" ]]; then
             echo "shasum mismatch for gfortran-darwin-${arch}-${type}"
             exit 1
         fi

tools/sha256sum.py

@@ -0,0 +1,13 @@
+#!/usr/bin/env python3
+
+import hashlib
+import sys
+ 
+filename = sys.argv[1]
+sha256_hash = hashlib.sha256()
+with open(filename,"rb") as f:
+    # Read and update hash string value in blocks
+    for byte_block in iter(lambda: f.read(32768),b""):
+        sha256_hash.update(byte_block)
+    print(sha256_hash.hexdigest())
+