Fix loading libcrypto on OSX Big Sur

It looks like Big Sur removed `libcrypto.dylib` as a file on the
filesystem, so loading it using `ctypes.find_library` fails which breaks
Kindle decryption. Now to load a dylib you need to attempt to load it
directly and the operating system will load the dylib from the OS' cache
or fail.

This fixes the problem by explicitly setting the path to libcrypto to
`/usr/lib/libcrypto.dylib` if `ctypes.find_library` does not find the
file, loading the dylib and raising an exception if it fails at that
point.

See saltstack/salt#5778 for more detailed info.

Closes #1369.

DeDRM_plugin/kindlekey.py

@@ -1183,8 +1183,12 @@ elif isosx:
 
         libcrypto = find_library('crypto')
         if libcrypto is None:
-            raise DrmException(u"libcrypto not found")
+            libcrypto = '/usr/lib/libcrypto.dylib'
+        try:
             libcrypto = CDLL(libcrypto)
+        except Exception as e:
+            raise DrmException(u"libcrypto not found: " % e)
+
 
         # From OpenSSL's crypto aes header
         #