Fix optimised_pow2_inplace() on Python 3.10 (GH-4103)
Fix optimised_pow2_inplace() on Python 3.10 (GH-4103)

Fix optimised_pow2_inplace() doctest on Python 3.10 because the error message changed.

Python 3.9 error message:
    unsupported operand type(s) for ** or pow(): 'int' and 'str'

Python 3.10 error message:
    unsupported operand type(s) for **=: 'int' and 'str'
scoder/cython,cython/cython,da-woods/cython,da-woods/cython,scoder/cython,cython/cython,cython/cython,da-woods/cython,cython/cython,da-woods/cython,scoder/cython,scoder/cython