Uniformize UTF-8 naming (#1115)

* Uniformize UTF-8 naming

Replace `utf8` -> `utf-8` everywhere.
It should have no impact, `utf8` is an alias of `utf-8` [1].

[1] https://github.com/python/cpython/blob/ee03bad25e83b00ba5fc2a0265b48c6286e6b3f7/Lib/encodings/aliases.py#L534

* Always specify the encoding

Let's be explicit over implicit. And prevent future warnings from PEP-597 [1].

[1] https://www.python.org/dev/peps/pep-0597/#using-the-default-encoding-is-a-common-mistake

* Update `UTF8` constant (`utf-8` -> `utf_8`)

* Remove default argument from `str.encode()` and `bytes.decode()`

* Clean-up

tests/test_cli.py

@@ -118,7 +118,7 @@ class TestItemParsing:
         # Parsed file fields
         assert 'file' in items.files
         assert (items.files['file'][1].read().strip().
-                decode('utf8') == FILE_CONTENT)
+                decode() == FILE_CONTENT)
 
     def test_multiple_file_fields_with_same_field_name(self):
         items = RequestItems.from_args([