Fix error can't open image files in Python3.

[youdiandai]

1.We tried to use your CRNN network for training, and the Python version we used was 3.7.2,
in the process of generating the LMDB file, I noticed that......an error will occur, Python
3 need to add a "b" flag bit when reading bit type data.
"""
with open(imagePath, 'r') as f:
imageBin = f.read()

"""

2. I noticed that there was an error when you directly pushed STR into LMDB when I put data
   into LMDB.LMDB should only accept data of bytes type, so we have to encode data asbyte type.

3.In the process of reading data from LMDB, since the data read out is bytes type, the
string operation cannot be carried out directly, and the bytes type decode needs to be
str.

4.The data sets we used is different form yours, so some of the characters in yout
"alphabets.pu"cannot be found and cause application error. I wrote a function to
generate the corresponding "apphabets.py" module according to the data sets wo imprvoe
compatibility.

[lironghua318]

for 3
there is already decode in

• "train.py", line 55, in val
  list_1.append(i.decode('utf-8','strict'))
• ''/lib/convert.py", line 43, in encode
  item = item.decode('utf-8', 'strict')

[youdiandai]

it been a long time since i create this merge.I cant rember it well.send me email if you need to discuss,my email : [email protected]