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/fixtures/__init__.py

@@ -1,6 +1,8 @@
 """Test data"""
 from pathlib import Path
 
+from httpie.constants import UTF8
+
 
 def patharg(path):
     """
@@ -23,9 +25,9 @@ JSON_FILE_PATH_ARG = patharg(JSON_FILE_PATH)
 # Strip because we don't want new lines in the data so that we can
 # easily count occurrences also when embedded in JSON (where the new
 # line would be escaped).
-FILE_CONTENT = FILE_PATH.read_text('utf8').strip()
+FILE_CONTENT = FILE_PATH.read_text(encoding=UTF8).strip()
 
 
-JSON_FILE_CONTENT = JSON_FILE_PATH.read_text('utf8')
+JSON_FILE_CONTENT = JSON_FILE_PATH.read_text(encoding=UTF8)
 BIN_FILE_CONTENT = BIN_FILE_PATH.read_bytes()
 UNICODE = FILE_CONTENT