CI/CD failed due to large int<->str conversions

[cheqianh]

Currently, the CI/CD for pypy fails due to a python int<>str conversion issue. Need a fix since it will block all future PRs.

example log,

...
...
amazon/ion/core.py:286: in value
    return self[2]()
amazon/ion/core.py:261: in __call__
    self.value = self.delegate()
amazon/ion/reader_text.py:550: in parse
    return parse_func(value, base)
/opt/hostedtoolcache/PyPy/3.8.15/x64/lib/pypy3.8/_functools.py:80: in __call__
    return self._func(*(self._args + fargs), **fkeywords)
amazon/ion/reader_text.py:558: in _base_10
    return parse_func(value)
/opt/hostedtoolcache/PyPy/3.8.15/x64/lib/pypy3.8/_pydecimal.py:566: in __new__
    self._int = str(int(intpart+fracpart))
E   ValueError: Exceeds the limit (4[30](https://github.com/cheqianh/ion-python/actions/runs/3751918469/jobs/6373480918#step:7:31)0) for integer string conversion: value has 8190 digits