*.py: Open files in binary mode and decode to UTF-8 explicitly if needed

On Windows in text mode files are opened with encoding=locale.getdefaultlocale()
which is CP125X instead of UTF-8 even if $PYTHONIOENCODING=UTF-8. This
way e.g. test_strings_print fail as:

    E           Failed: not equal:
    E           Expected:
    E               print(qq(b)): "привет αβγ b"
    E               print(qq(u)): "привет αβγ u"
    E           Got:
    E               print(qq(b)): "привет αβγ b"
    E               print(qq(u)): "привет αβγ u"

where "Expected" was read from golang/testprog/golang_test_str.txt and
decoded wrongly.

-> Fix it by always opening files for reading in binary mode and
utf8-decoding manually, if needed, everywhere.

golang/golang_test.py

@@ -1889,8 +1889,10 @@ def test_fmtargspec():
 
 
 # readfile returns content of file @path.
-def readfile(path):
-    with open(path, "r") as f:
+def readfile(path): # -> bytes
+    # on windows in text mode files are opened with encoding=locale.getdefaultlocale()
+    # which is CP125X instead of UTF-8. -> manually decode as 'UTF-8'
+    with open(path, "rb") as f:
         return f.read()
 
 # abbrev_home returns path with user home prefix abbreviated with ~.

gpython/__init__.py

 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
-# Copyright (C) 2018-2021  Nexedi SA and Contributors.
+# Copyright (C) 2018-2023  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -439,13 +439,13 @@ def main():
 
 # _is_buildout_script returns whether file @path is generated as python buildout script.
 def _is_buildout_script(path):
-    with open(path, 'r') as f:
+    with open(path, 'rb') as f:
         src = f.read()
     # buildout injects the following prologues into python scripts:
     #   sys.path[0:0] = [
     #     ...
     #   ]
-    return ('\nsys.path[0:0] = [\n' in src)
+    return (b'\nsys.path[0:0] = [\n' in src)
 
 
 # _IGetOpt provides getopt-style incremental options parsing.

setup.py

@@ -26,9 +26,12 @@ from os.path import dirname, join
 import sys, os, re
 
 # read file content
-def readfile(path):
-    with open(path, 'r') as f:
-        return f.read()
+def readfile(path): # -> str
+    with open(path, 'rb') as f:
+        data = f.read()
+        if not isinstance(data, str):   # py3
+            data = data.decode('utf-8')
+        return data
 
 # reuse golang.pyx.build to build pygolang dso and extensions.
 # we have to be careful and inject synthetic golang package in order to be