fixup!  * Fix escaping chars in FormGroup title  * Use lxml to produce XML view  * Update XMLToForm to follow escaping changes

some elements were double escaped.

See merge request nexedi/erp5!1621

It seems to accept int, but version is supposed to be a string.

See merge request !1671

a modified version from nexedi/erp5$366
that supports patterns like
DateTime(2001, 02, 03) => DateTime(2001, 2, 3)
and not DateTime(2001, 0o2, 0o3)

Javascript does not support big integers, so the backend have to be
careful not to return too large numbers in the json. Nowadays there
is bigint, but not equivalent for decimal numbers, so the backend
tries to detect if numbers are too big and returns them as string
in that case - which is better than returning a different number
because it displays correctly in the result table. The pivot table
will still wrong, but at least the table of results is OK.

use sax.xml API which gives us the first parser available, instead of
using a specific parser that happens to come from pyxml

serialize as json instead of using xml.marshal.generic which was
provided by PyXML

See merge request nexedi/erp5!1668