Sign change when doing math gives invalid number

[michi12321]

Bug:

>>> d = tomlkit.parse('x=-3.1415')
>>> d['x'] *= -1
>>> print(tomlkit.dumps(d))

returns:

a=+3.1415

>>> d['x'] *= -1
>>> print(tomlkit.dumps(d))

returns:

a=--3.1415

Subsequent parsing raises tomlkit.exceptions.InvalidNumberError.

Of course there is an easy workaround using type conversion: d['x'] = float(d['x'])*-1.