Avoid a warning about regex flags in Python 3.6
Avoid a warning about regex flags in Python 3.6

Python 3.6 deprecated using regex flag syntax (?x) to set flags in the
middle of a pattern. Now you can only use it at the start of a pattern.
Fortunately that same release added a new scoped-flag syntax, (?x:).

(Wait, you say, it looks like our code *does* set flags at the start of
the pattern? That's true, but the markdown module includes our regex in the
middle of a larger one, so it's not actually at the start.)

Fixes this warning (and a couple similar ones):

DeprecationWarning: Flags not at the start of the expression '^(.?)(?x) \[ ([^]]' (truncated)
    self.compiled_re = re.compile(r"^(.?)%s(.)$" % pattern

veekun/pokedex,veekun/pokedex