Reading the documentation, I don't understand how this is not possible : 

# get the max utc timestamp
ts = datetime.max.replace(tzinfo=timezone.utc).timestamp()

# similarly 
ts2 = datetime(9999, 12, 31, 23, 59, 59, 999999, tzinfo=timezone.utc).timestamp()

# timestamp value 253402300800 seems correct
# converting back to timestamp is impossible, these all fail : 

dt = datetime.fromtimestamp(ts, tz=timezone.utc)
dt = datetime.utcfromtimestamp(ts)

It should be possible to get a datetime back from the initially converted timestamp, no?

Please show us how they fail.

a ValueError is raised : 

ValueError: year 10000 is out of range 

on 

dt = datetime.fromtimestamp(ts, tz=timezone.utc)

or 

dt = datetime.utcfromtimestamp(ts)

I see this in the python source code being tested (datetimetester.py), so I guess it is a rounding problem : 

# maximum timestamp: set seconds to zero to avoid rounding issues
        max_dt = self.theclass.max.replace(tzinfo=timezone.utc,
                                           second=0, microsecond=0)
        max_ts = max_dt.timestamp()
        # date 9999-12-31 23:59:00+00:00: timestamp 253402300740
        self.assertEqual(self.theclass.fromtimestamp(max_ts, tz=timezone.utc),
                         max_dt)

Probably so. You could step through the code to make sure that's what's going on.

I looked at this a bit more in detail.
What happens it the following, starting in the datetime fromtimestamp fragment :

converter = _time.gmtime if utc else _time.localtime
y, m, d, hh, mm, ss, weekday, jday, dst = converter(t)

That will call the system gmtime_r(), which indeed returns Sat Jan  1 00:00:00 10000

I have not looked at the cpp implementation for that method and I am not sure if this is something that can be improved.