bpo-43651: PEP 597: Fix EncodingWarning in some tests (GH-25171)

* Fix test_float
* Fix _osx_support
* Fix test_fstring
* Fix test_gc
* Fix test_gzip
* Fix test_hashlib
* Fix unrelated whitespace issue

Co-authored-by: Ned Deily <nad@python.org>
diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index 79e123f..296cb05 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -1110,7 +1110,7 @@ def test_filename_in_syntaxerror(self):
         # see issue 38964
         with temp_cwd() as cwd:
             file_path = os.path.join(cwd, 't.py')
-            with open(file_path, 'w') as f:
+            with open(file_path, 'w', encoding="utf-8") as f:
                 f.write('f"{a b}"') # This generates a SyntaxError
             _, _, stderr = assert_python_failure(file_path,
                                                  PYTHONIOENCODING='ascii')