Fix t_ocr.py for PNG image. (#4625)

### What problem does this PR solve?
#4586

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

deepdoc/vision/__init__.py

 #  See the License for the specific language governing permissions and
 #  limitations under the License.
 #
+import io
 
 import pdfplumber
 
             pdf_pages(fnm)
             return
         try:
-            images.append(Image.open(fnm))
+            fp = open(fnm, 'rb')
+            binary = fp.read()
+            fp.close()
+            images.append(Image.open(io.BytesIO(binary)).convert('RGB'))
             outputs.append(os.path.split(fnm)[-1])
         except Exception:
             traceback.print_exc()