Switch to defusedxml for xml parsing

xml.etree.ElementTree.fromstring is considered insecure, see:
https://docs.python.org/3/library/xml.etree.elementtree.html

The defusedxml package contains several Python-only workarounds and
fixes for denial of service and other vulnerabilities in Python's XML
libraries: https://github.com/tiran/defusedxml

Fixes #670

app/request.py

@@ -1,6 +1,6 @@
 from app.models.config import Config
 from datetime import datetime
-import xml.etree.ElementTree as ET
+from defusedxml import ElementTree as ET
 import random
 import requests
 from requests import Response, ConnectionError

requirements.txt

@@ -6,6 +6,7 @@ cffi==1.15.0
 chardet==3.0.4
 click==8.0.3
 cryptography==3.3.2
+defusedxml==0.7.1
 Flask==1.1.1
 Flask-Session==0.4.0
 idna==2.9

setup.cfg

@@ -19,6 +19,7 @@ include_package_data = True
 install_requires=
     beautifulsoup4
     cryptography
+    defusedxml
     Flask
     Flask-Session
     python-dotenv