Typing corrections (#109)

* Correct typing of SMBus.fd

At initialization SMBus.fd is None making the correct typing
Optional[int].

* Correct typing of SMBus.enable_pec

enable_pec's enable parameter has a default value that was not captured
in the typing information.

* Correct typing of SMBus.force

SMBus.force is never set to None unless a user manually sets it as such.
Additionally, no logic checks if SMBus.force is None, only if it is
True, making False and None equivalent in this case.

smbus2/smbus2.pyi

@@ -80,10 +80,10 @@ class i2c_rdwr_ioctl_data(Structure):
     def create(*i2c_msg_instances: Sequence[i2c_msg]) -> "i2c_rdwr_ioctl_data": ...
 
 class SMBus:
-    fd: int = ...
+    fd: Optional[int] = ...
     funcs: I2cFunc = ...
     address: Optional[int] = ...
-    force: Optional[bool] = ...
+    force: bool = ...
     pec: int = ...
     def __init__(
         self, bus: _UnionT[None, int, str] = ..., force: bool = ...
@@ -97,7 +97,7 @@ class SMBus:
     ) -> None: ...
     def open(self, bus: _UnionT[int, str]) -> None: ...
     def close(self) -> None: ...
-    def enable_pec(self, enable: bool) -> None: ...
+    def enable_pec(self, enable: bool = ...) -> None: ...
     def write_quick(self, i2c_addr: int, force: Optional[bool] = ...) -> None: ...
     def read_byte(self, i2c_addr: int, force: Optional[bool] = ...) -> int: ...
     def write_byte(