Remove redundant steps in alabel()

[elliotwutingfeng]

Proposed change

I found some portions of alabel() which are either unreachable or redundant.

---

if not label:
    raise IDNAError('No Input')

is not reachable because an empty string would be caught by ulabel(label_bytes) in the preceding try-block.

---

label = str(label)

is redundant as label is already a str, if it was not a str it would have been caught by the preceding try-block.

---

We can simplify label_bytes as _alabel_prefix + _punycode(label).