Remove a hack for mypy<0.790

We have upgraded to mypy 0.790 in #26066, which brings us
python/typeshed#4146.

Unfortunately, we still have to use io.open instead of the built-in open
to make mypy happy in Python 2, because open() is still typed as
BinaryIO unconditionally in Python 2.

Author: Hexcles

tools/ci/tc/github_checks_output.py

@@ -1,9 +1,10 @@
+import io
+
 MYPY = False
 if MYPY:
     # MYPY is set to True when run under Mypy.
-    from typing import Any
-    from typing import Optional
-    from typing import Text
+    from typing import Optional, Text, TextIO
+
 
 class GitHubChecksOutputter(object):
     """Provides a method to output data to be shown in the GitHub Checks UI.
@@ -19,16 +20,17 @@ def __init__(self, path):
         self.path = path
 
     def output(self, line):
-        # type: (Any) -> None
-        # TODO(stephenmcgruer): Once mypy 0.790 is released, we can change this
-        # to AnyStr, as that release teaches mypy about the mode flags of open.
-        # See https://github.com/python/typeshed/pull/4146
-        with open(self.path, 'a') as f:
+        # type: (Text) -> None
+        # mypy always casts open() as BinaryIO in Python 2 regardless of the mode.
+        # TODO(Hexcles): use the built-in open() when we are Py3-only.
+        with io.open(self.path, mode="a") as f:
             f.write(line)
             f.write('\n')
 
 
 __outputter = None
+
+
 def get_gh_checks_outputter(filepath):
     # type: (Optional[Text]) -> Optional[GitHubChecksOutputter]
     """Return the outputter for GitHub Checks output, if enabled.