Fix 'expected str instance, bytes found' error when compute patches

When building an incremental OTA package, if the compress_target=True
parameter is passed in when calculating a block, an exception will occur
in the subsequent join operation.

The reason is that Python 3 no longer allows str type join bytes type.

Bug: None
Test: Build Incremental OTA package

Change-Id: I6d556f9905b7ab75b70d3785334d71d5a6e5479b
Signed-off-by: luoqiangwei1 <luoqiangwei1@xiaomi.com>
diff --git a/tools/releasetools/blockimgdiff.py b/tools/releasetools/blockimgdiff.py
index 211182a..8087fcd 100644
--- a/tools/releasetools/blockimgdiff.py
+++ b/tools/releasetools/blockimgdiff.py
@@ -1171,7 +1171,7 @@
           try:
             # Compresses with the default level
             compress_obj = zlib.compressobj(6, zlib.DEFLATED, -zlib.MAX_WBITS)
-            compressed_data = (compress_obj.compress("".join(tgt_data))
+            compressed_data = (compress_obj.compress(b"".join(tgt_data))
                                + compress_obj.flush())
             compressed_size = len(compressed_data)
           except zlib.error as e: