sort_reexports breaks __all__ symbol

[jugmac00]

repro

• install isort from current head ( c6a4196 )
• create test.py with one line __all__ = ["b", "c", "a"]
• create either config file with sort_reexports = true and run isort test.py or run isort --sort_reexports test.py

The above line in test.py will be changed into...

___all__ = ["a", "b", "c"]

Please note the third underline character at the beginning of the symbol.

Python 3.8.10 on Ubuntu 20.04