Add type annotations to compat.to_str() and compat.to_bytes().

- I'm not sure if these functions are truly necessary if the library no longer supports Python 2, but the use of it with `encoding` is prevalant enough around the library that it seems worth typing.
- The overloads are necessary to account for None being a passthrough value.
 - Two cases of this are `ParseResult.copy_with()` and `ParseResultBytes.copy_with()`. The `attrs_dict` dictionary in those methods is allowed to have None, and None is allowed for all of the component parameters (from what I can tell). Thus, whether intentional or not, `compat.to_(bytes|str)()`'s ability to let values pass through without modification, so long as they aren't bytes, is depended upon.

src/rfc3986/compat.py

@@ -12,21 +12,48 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 """Compatibility module for Python 2 and 3 support."""
+import typing as t
 
 __all__ = (
     "to_bytes",
     "to_str",
 )
 
 
-def to_str(b, encoding="utf-8"):
+@t.overload
+def to_str(b: t.Union[str, bytes], encoding: str = "utf-8") -> str:
+    ...
+
+
+@t.overload
+def to_str(b: None, encoding: str = "utf-8") -> None:
+    ...
+
+
+def to_str(
+    b: t.Optional[t.Union[str, bytes]],
+    encoding: str = "utf-8",
+) -> t.Optional[str]:
     """Ensure that b is text in the specified encoding."""
     if hasattr(b, "decode") and not isinstance(b, str):
         b = b.decode(encoding)
     return b
 
 
-def to_bytes(s, encoding="utf-8"):
+@t.overload
+def to_bytes(s: t.Union[str, bytes], encoding: str = "utf-8") -> bytes:
+    ...
+
+
+@t.overload
+def to_bytes(s: None, encoding: str = "utf-8") -> None:
+    ...
+
+
+def to_bytes(
+    s: t.Optional[t.Union[str, bytes]],
+    encoding: str = "utf-8",
+) -> t.Optional[bytes]:
     """Ensure that s is converted to bytes from the encoding."""
     if hasattr(s, "encode") and not isinstance(s, bytes):
         s = s.encode(encoding)