I created this code to get all excel files in a folder and make a csv file to every sheet in every file. This script works fine, but sometimes the last file converted still locked by python on file system. Can anyone help me to understand what’s happening?

import sys
from os import listdir
from os.path import isfile, join
import pandas as pd
import csv
import re

def removeEspecialCharacters(obj):
    if isinstance(obj, str) :
        retorno = re.sub('[(\x90|\x8F)]','',obj).replace("\r","").replace("\n","")
    else:
        retorno = obj
        
    return retorno

myFolder = r'C:\Users\myuser\Downloads\ConvertFilesToCsv'
myFiles = [f for f in listdir(myFolder) if isfile(join(myFolder, f))]

for x in range(len(myFiles)):   
    if (myFiles[x].lower().endswith('.xls') or myFiles[x].lower().endswith('.xlsx') or myFiles[x].lower().endswith('.xlsb')):
        print('Converting file: '+myFiles[x]);
        if (myFiles[x].lower().endswith('.xlsb')):
            file = pd.ExcelFile(myFolder+'\\'+myFiles[x], engine="pyxlsb")
        else:
            file = pd.ExcelFile(myFolder+'\\'+myFiles[x])
    
        for mySheetName in file.sheet_names:
            df = pd.read_excel(file, sheet_name=mySheetName)
            df = df.applymap(removeEspecialCharacters)
            csvFileName = myFolder+'\\'+myFiles[x].replace('.xlsx','').replace('.xlsb','').replace('.xls','')+'_'+mySheetName+'.csv'
            df.to_csv(csvFileName,encoding='utf-8-sig',index=False,sep=",",quoting=csv.QUOTE_NONNUMERIC,quotechar="\"",escapechar="\"",decimal=".",date_format="%Y-%m-%d")#,quotechar="\"', escapechar="\\")
        file.close()
        file=""