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] ee03bad25e/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

httpie/cli/argparser.py

@@ -301,7 +301,7 @@ class HTTPieArgumentParser(argparse.ArgumentParser):
         """
         self._ensure_one_data_source(self.has_stdin_data, self.args.data,
                                      self.args.files)
-        self.args.data = data.encode('utf-8')
+        self.args.data = data.encode()
 
     def _ensure_one_data_source(self, *other_sources):
         """There can only be one source of input request data.

httpie/cli/requestitems.py

@@ -144,7 +144,7 @@ def load_text_file(item: KeyValueArg) -> str:
     except UnicodeDecodeError:
         raise ParseError(
             f'{item.orig!r}: cannot embed the content of {item.value!r},'
-            ' not a UTF8 or ASCII-encoded text file'
+            ' not a UTF-8 or ASCII-encoded text file'
         )