Always set encoding when reading setup.cfg

This should fix an issue on Window where the `setup.cfg` can contain
non-ascii characters, and Python is trying to use cp1252. This should be
safe as utf-8 is compatible with cp1252.

It's possible I'll eat my words and some other platform breaks with this
change, but let's try.

Closes #294.

autoflake.py

@@ -20,6 +20,7 @@
 # TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
 # SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 """Removes unused imports and unused variables as reported by pyflakes."""
+
 from __future__ import annotations
 
 import ast
@@ -1208,7 +1209,7 @@ def process_config_file(config_file_path: str) -> MutableMapping[str, Any] | Non
     import configparser
 
     reader = configparser.ConfigParser()
-    reader.read(config_file_path)
+    reader.read(config_file_path, encoding="utf-8")
     if not reader.has_section("autoflake"):
         return None