fix: enum generation if it does contain only equality-sign

[dswistowski]

in case of type like:

CREATE TYPE operator AS ENUM (
    '=',
    '>',
    '<',
    '>=',
    '<='
);

current plugin will generate wrong python code:

class Operator(str, enum.Enum):
    = "="
    = ">"
    = "<"
    = ">="
    = "<="

after this change plugin will generate valid enum:

class Operator(str, enum.Enum):
    EQ = "="
    GT = ">"
    LT = "<"
    GTEQ = ">="
    LTEQ = "<="