TypeError: write() argument must be str, not bytes python 3.7.1

[Faetheless]

Here's my code:

`import csv
from fdfgen import forge_fdf
import os
import sys

sys.path.insert(0, os.getcwd())
filename_prefix = "NVC"
csv_file = "nvc.csv"
pdf_file = "NVC.pdf"
tmp_file = "tmp.fdf"
output_folder = './output/'

def process_csv(file):
headers = []
data = []
csv_data = csv.reader(open(file))
for i, row in enumerate(csv_data):
if i == 0:
headers = row
continue;
field = []
for i in range(len(headers)):
field.append((headers[i], row[i]))
data.append(field)
return data

def form_fill(fields):
fdf = forge_fdf("",fields,[],[],[])
fdf_file = open(tmp_file,"wb")
fdf_file.write(fdf)
fdf_file.close()
output_file = '{0}{1} {2}.pdf'.format(output_folder, filename_prefix, fields[1][1])
cmd = 'pdftk "{0}" fill_form "{1}" output "{2}" dont_ask'.format(pdf_file, tmp_file, output_file)
os.system(cmd)
os.remove(tmp_file)

data = process_csv(csv_file)
print('Generating Forms:')
print('-----------------------')
for i in data:
if i[0][1] == 'Yes':
continue
print('{0} {1} created...'.format(filename_prefix, i[1][1]))
form_fill(i)`

every time I run this it gives me the above type error. I've tried changing variables, but it doesn't seem to matter any time I try to write the fdf file I get that error.