Regression in unescaping backslash in attribute

Regression in 3.4 - c0f6e5a is the first bad commit

Reproduction code:

import markdown

s = r'''
<img src="..\..\foo.png">
'''
print(repr(markdown.markdown(s)))

Output before vs after:

'<p><img src="..\\..\\foo.png"></p>'

'<p><img src="..\\\x0246\x03.\\foo.png"></p>'

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