Handle integers larger than a float in scientific

[itzzdev09]

Fixes #

Changes proposed in this pull request:

• scientific falls back to Decimal formatting when float() overflows, so an integer larger than the float range no longer raises OverflowError
• Behaviour for every other input (floats, strings, NaN/Inf, small ints) is unchanged, since Decimal is only used on the overflow path
• Tests for 10**400, its negative, a precision argument and precision 0, plus 0 to pin the unchanged path

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Same family as #420: scientific converts with float() first, which raises OverflowError for an integer beyond the float range and is not covered by the except (ValueError, TypeError) clause.

>>> humanize.scientific(10**400)
OverflowError: int too large to convert to float

After this change it returns '1.00 x 10⁴⁰⁰'.

Decimal is not used for all integers because format(Decimal(0), ".2e") gives 0.00e+2, which would break scientific(0).