[Python] Use correct type for str with None

Otherwise mypy will correctly flag code like this

def __init__(self):
  self.fooBar = None  # type: Optional[str]

error: Incompatible types in assignment (expression has type "None", variable has type "str")

Author: fliiiix

src/idl_gen_python.cpp

@@ -1628,7 +1628,7 @@ class PythonGenerator : public BaseGenerator {
     } else if (IsInteger(base_type)) {
       return "int";
     } else if (base_type == BASE_TYPE_STRING) {
-      return "str";
+      return "Optional[str]";
     } else {
       FLATBUFFERS_ASSERT(false && "base_type is not a scalar or string type.");
       return "";
@@ -1769,7 +1769,7 @@ class PythonGenerator : public BaseGenerator {
       }
 
       const auto default_value = GetDefaultValue(field);
-      // Wrties the init statement.
+      // Writes the init statement.
       const auto field_field = namer_.Field(field);
       code += GenIndents(2) + "self." + field_field + " = " + default_value +
               "  # type: " + field_type;

tests/MyGame/Example/Monster.py

@@ -1407,13 +1407,13 @@ def __init__(self):
         self.pos = None  # type: Optional[MyGame.Example.Vec3.Vec3T]
         self.mana = 150  # type: int
         self.hp = 100  # type: int
-        self.name = None  # type: str
+        self.name = None  # type: Optional[str]
         self.inventory = None  # type: List[int]
         self.color = 8  # type: int
         self.testType = 0  # type: int
         self.test = None  # type: Union[None, MyGame.Example.Monster.MonsterT, MyGame.Example.TestSimpleTableWithEnum.TestSimpleTableWithEnumT, MyGame.Example2.Monster.MonsterT]
         self.test4 = None  # type: List[MyGame.Example.Test.TestT]
-        self.testarrayofstring = None  # type: List[str]
+        self.testarrayofstring = None  # type: List[Optional[str]]
         self.testarrayoftables = None  # type: List[MyGame.Example.Monster.MonsterT]
         self.enemy = None  # type: Optional[MyGame.Example.Monster.MonsterT]
         self.testnestedflatbuffer = None  # type: List[int]
@@ -1431,7 +1431,7 @@ def __init__(self):
         self.testf = 3.14159  # type: float
         self.testf2 = 3.0  # type: float
         self.testf3 = 0.0  # type: float
-        self.testarrayofstring2 = None  # type: List[str]
+        self.testarrayofstring2 = None  # type: List[Optional[str]]
         self.testarrayofsortedstruct = None  # type: List[MyGame.Example.Ability.AbilityT]
         self.flex = None  # type: List[int]
         self.test5 = None  # type: List[MyGame.Example.Test.TestT]

tests/MyGame/Example/NestedUnion/NestedUnionTest.py

@@ -105,7 +105,7 @@ class NestedUnionTestT(object):
 
     # NestedUnionTestT
     def __init__(self):
-        self.name = None  # type: str
+        self.name = None  # type: Optional[str]
         self.dataType = 0  # type: int
         self.data = None  # type: Union[None, MyGame.Example.NestedUnion.Vec3.Vec3T, MyGame.Example.NestedUnion.TestSimpleTableWithEnum.TestSimpleTableWithEnumT]
         self.id = 0  # type: int

tests/MyGame/Example/Stat.py

@@ -84,7 +84,7 @@ class StatT(object):
 
     # StatT
     def __init__(self):
-        self.id = None  # type: str
+        self.id = None  # type: Optional[str]
         self.val = 0  # type: int
         self.count = 0  # type: int
 

tests/monster_test_generated.py

@@ -728,7 +728,7 @@ class StatT(object):
 
     # StatT
     def __init__(self):
-        self.id = None  # type: str
+        self.id = None  # type: Optional[str]
         self.val = 0  # type: int
         self.count = 0  # type: int
 
@@ -1973,13 +1973,13 @@ def __init__(self):
         self.pos = None  # type: Optional[Vec3T]
         self.mana = 150  # type: int
         self.hp = 100  # type: int
-        self.name = None  # type: str
+        self.name = None  # type: Optional[str]
         self.inventory = None  # type: List[int]
         self.color = 8  # type: int
         self.testType = 0  # type: int
         self.test = None  # type: Union[None, MonsterT, TestSimpleTableWithEnumT, MonsterT]
         self.test4 = None  # type: List[TestT]
-        self.testarrayofstring = None  # type: List[str]
+        self.testarrayofstring = None  # type: List[Optional[str]]
         self.testarrayoftables = None  # type: List[MonsterT]
         self.enemy = None  # type: Optional[MonsterT]
         self.testnestedflatbuffer = None  # type: List[int]
@@ -1997,7 +1997,7 @@ def __init__(self):
         self.testf = 3.14159  # type: float
         self.testf2 = 3.0  # type: float
         self.testf3 = 0.0  # type: float
-        self.testarrayofstring2 = None  # type: List[str]
+        self.testarrayofstring2 = None  # type: List[Optional[str]]
         self.testarrayofsortedstruct = None  # type: List[AbilityT]
         self.flex = None  # type: List[int]
         self.test5 = None  # type: List[TestT]