astropy: Leap second tests fail if system clock is in the future (2021-06-04)

Description

As part of my work on reproducible builds for openSUSE, I check that software still gives identical build results in the future. The usual offset is +15 years, because that is how long I expect some software will be used in some places. This showed up failing tests in our python-astropy package build. See https://reproducible-builds.org/ for why this matters.

Expected behavior

Software should be able to build anytime.

Actual behavior

tests fail with

astropy/utils/iers/iers.py:1021: in auto_open
    warn('leap-second file is expired.', IERSStaleWarning)
E   astropy.utils.iers.iers.IERSStaleWarning: leap-second file is expired.

That check already fails before 2021-06-04

and when dropping that expiry check, there is another problem with Time.now()

astropy/time/core.py:583: in now
    return cls(val=dtnow, format='datetime', scale='utc')
astropy/time/core.py:430: in __init__ 
    self._init_from_vals(val, val2, format, scale, copy,
astropy/time/core.py:485: in _init_from_vals
    self._time = self._get_time_fmt(val, val2, format, scale,
astropy/time/core.py:543: in _get_time_fmt
    return cls(val, val2, scale, precision, in_subfmt, out_subfmt)
astropy/time/formats.py:148: in __init__ 
    self.set_jds(val1, val2)
astropy/time/formats.py:831: in set_jds
    jd1, jd2 = erfa.dtf2d(self.scale.upper().encode('ascii'),
astropy/_erfa/core.py:15703: in dtf2d 
    check_errwarn(c_retval, 'dtf2d') 

Steps to Reproduce

1. set system clock to 2035
2. run tests: py.test-3.8

System Details

Linux-5.6.6-1-default-x86_64-with-glibc2.2.5 Python 3.8.2 (default, Mar 05 2020, 18:58:42) [GCC] Numpy 1.18.3 astropy 4.0.1.post1 Scipy 1.4.1