remove xss leave some extra attribute

[nattapong ratanasuwan]

from lxml.html.clean import Cleaner

cleaner = Cleaner(
    style=True,
    links=True,
    add_nofollow=True,
    page_structure=False,
    safe_attrs_only=False,
)

raw_description = '<b onmouseover=alert("XSS testing!")>Test</b>'
cleaner.clean_html(raw_description)

actual
```
<b testing>Test</b>\n
```
expect
```
<b>Test</b>
```

it should not have attribute `testing`