Support PEP 515 -- Underscores in Numeric Literals
In Python 3.6, Python added support for using underscores in numeric literals. For example,
x = 1000000 and
x = 1_000_000 are equivalent.
x = 1_000_000 currently throws an error.
It's likely that we want to enable this for tako in all supported Python versions, rather than limiting it to tako instances running on Python 3.6.