Added defusedxml to parse untrusted XML data

[mstfbl]

This PR fixes an issue pointed out by Bandit w.r.t. using the xml library to parse untrusted data, by using defusedxml instead.

Bandit output:

>> Issue: [B314:blacklist] Using xml.etree.ElementTree.parse to parse untrusted XML data is known to be vulnerable to XML attacks. Replace xml.etree.ElementTree.parse with its defusedxml equivalent function or make sure defusedxml.defuse_stdlib() is called
   Severity: Medium   Confidence: High
   Location: ./torchtext/data/datasets_utils.py:24
   More Info: https://bandit.readthedocs.io/en/latest/blacklists/blacklist_calls.html#b313-b320-xml-bad-elementtree
23	    with codecs.open(f_txt, mode='w', encoding='utf-8') as fd_txt:
24	        root = ET.parse(f_xml).getroot()[0]
25	        for doc in root.findall('doc'):

>> Issue: [B314:blacklist] Using xml.etree.ElementTree.parse to parse untrusted XML data is known to be vulnerable to XML attacks. Replace xml.etree.ElementTree.parse with its defusedxml equivalent function or make sure defusedxml.defuse_stdlib() is called
   Severity: Medium   Confidence: High
   Location: ./torchtext/legacy/datasets/translation.py:169
   More Info: https://bandit.readthedocs.io/en/latest/blacklists/blacklist_calls.html#b313-b320-xml-bad-elementtree
168	            with codecs.open(f_txt, mode='w', encoding='utf-8') as fd_txt:
169	                root = ET.parse(f_xml).getroot()[0]
170	                for doc in root.findall('doc'):

[codecov]

Codecov Report

Merging #1279 (3ea63f7) into master (c37f8a0) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1279      +/-   ##
==========================================
+ Coverage   78.80%   78.84%   +0.03%     
==========================================
  Files          67       67              
  Lines        3624     3630       +6     
==========================================
+ Hits         2856     2862       +6     
  Misses        768      768              

Impacted Files	Coverage Δ
torchtext/data/datasets_utils.py	90.76% <100.00%> (+0.14%)	⬆️
torchtext/legacy/datasets/translation.py	37.23% <100.00%> (+2.06%)	⬆️

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