[BUG] Brython doesn't find attributes when a JS instance inherit from `HTMLElement`

[denis-migdal]

Be 2 JS classes having both a content property :

class A extends HTMLElement {
    get content() { return "ok" }
}
customElements.define("a-a", A)
window.A = new A()

class B {
    get content() { return "ok" }
}
window.B = new B() 

Python "content" in operator will work for class B but not for class A :

from browser import window

print( "content" in window.A, type(window.A) )
print( "content" in window.B, type(window.B) )

False <class 'DOMNode'>
True <class 'JSObject'>