背景

Djangoのバージョンを1.11.1から2.2へあげると、バイト文字列で保存したTextFieldsの値が取り出した時に、
b'...'と皮まで保存されるようになっていた。

結論

decodeを使ってstring型に変換していたのがstr()を使ってstring型へ変更するようになっていた。
str()を使うと改行文字がエスケープされたり、b''まで文字列に含まれたりする。

参考

version 1.11.1

s = s.decode(encoding, errors)
https://docs.djangoproject.com/ja/1.11/_modules/django/db/models/fields/

django.db.models.fields.py

from django.utils.encoding import (
    force_bytes, force_text, python_2_unicode_compatible, smart_text,
)

class TextField(Field):
    description = _("Text")

    def to_python(self, value):
        if isinstance(value, six.string_types) or value is None:
            return value
        return force_text(value)

django.utils.encoding.py

def force_text(s, encoding='utf-8', strings_only=False, errors='strict'):
    """
    Similar to smart_text, except that lazy instances are resolved to
    strings, rather than kept as lazy objects.

    If strings_only is True, don't convert (some) non-string-like objects.
    """
    # Handle the common case first for performance reasons.
    if issubclass(type(s), six.text_type):
        return s
    if strings_only and is_protected_type(s):
        return s
    try:
        if not issubclass(type(s), six.string_types):
            if six.PY3:
                if isinstance(s, bytes):
                    s = six.text_type(s, encoding, errors)
                else:
                    s = six.text_type(s)
            elif hasattr(s, '__unicode__'):
                s = six.text_type(s)
            else:
                s = six.text_type(bytes(s), encoding, errors)
        else:
            # Note: We use .decode() here, instead of six.text_type(s, encoding,
            # errors), so that if s is a SafeBytes, it ends up being a
            # SafeText at the end.
            s = s.decode(encoding, errors)
    except UnicodeDecodeError as e:
        if not isinstance(s, Exception):
            raise DjangoUnicodeDecodeError(s, *e.args)
        else:
            # If we get to here, the caller has passed in an Exception
            # subclass populated with non-ASCII bytestring data without a
            # working unicode method. Try to handle this without raising a
            # further exception by individually forcing the exception args
            # to unicode.
            s = ' '.join(force_text(arg, encoding, strings_only, errors)
                         for arg in s)
    return s

version 2.2

django.db.models.fields.py

class TextField(Field):
    description = _("Text")

    def to_python(self, value):
        if isinstance(value, str) or value is None:
            return value
        return str(value)