Fix mypy complains

src/escpos/printer/network.py

@@ -53,7 +53,7 @@ class Network(Escpos):
 
     def __init__(
         self,
-        host: str = None,
+        host: str = "",
         port: int = 9100,
         timeout: Union[int, float] = 60,
         *args,

src/escpos/printer/serial.py

@@ -10,7 +10,7 @@
 
 
 import functools
-from typing import Union
+from typing import Union, Optional
 
 from ..escpos import Escpos
 
@@ -78,8 +78,8 @@ class Serial(Escpos):
         baudrate: int = 9600,
         bytesize: int = 8,
         timeout: Union[int, float] = 1,
-        parity=None,  # type: str?
+        parity: Optional[str] = None,
-        stopbits=None,  # type: int?
+        stopbits: Optional[int] = None,
         xonxoff: bool = False,
         dsrdtr: bool = True,
         *args,

src/escpos/printer/usb.py

@@ -77,8 +77,8 @@ class Usb(Escpos):
         idProduct: str = "",
         usb_args: Dict[str, str] = {},
         timeout: Union[int, float] = 0,
-        in_ep=0x82,  # type: int?
+        in_ep: int = 0x82,
-        out_ep=0x01,  # type: int?
+        out_ep: int = 0x01,
         *args,
         **kwargs
     ):