prometheus_speedtest: Fix build failure by PEP 597 warning in pylint.

[PEP 597](https://www.python.org/dev/peps/pep-0597/) adds an additional warning
when using open() without specifying an explicit encoding. This was added to
pylint in pylint-dev/pylint#3826 and eventually broke our
build as a new warning.

setup.py

@@ -14,7 +14,7 @@ def read_file(rel_path):
       rel_path: relative file path, string.
     """
     here = os.path.abspath(os.path.dirname(__file__))
-    with open(os.path.join(here, rel_path), 'r') as rel_file:
+    with open(os.path.join(here, rel_path), 'r', encoding='utf-8') as rel_file:
         return rel_file.read().strip()