meson: use encoding=utf-8 when reading/writing files in helper script

Fixes errors such as

Traceback (most recent call last):
  File "C:\Users\...\cairo\test\make-cairo-test-constructors.py", line 19, in <module>
    for l in f.readlines():
  File "c:\python39\lib\encodings\cp1253.py", line 23, in decode
    return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x98 in position 6694: character maps to <undefined>

on non-English-language Windows locales/installations.

boilerplate/make-cairo-boilerplate-constructors.py

@@ -16,12 +16,12 @@ if __name__=='__main__':
     match_boilerplate_name = re.compile(r'^CAIRO_BOILERPLATE.*\((.*),.*')
 
     for fname in args.input:
-        with open(fname, 'r') as f:
+        with open(fname, 'r', encoding='utf-8') as f:
             for l in f.readlines():
                 if match_boilerplate_line.match(l):
                     boilerplate_names.append(match_boilerplate_name.match(l).group(1))
 
-    with open(args.output, 'w') as f:
+    with open(args.output, 'w', encoding='utf-8') as f:
         f.write('/* WARNING: Autogenerated file - see %s! */\n\n' % sys.argv[0])
         f.write('#include "cairo-boilerplate-private.h"\n\n')
         f.write('void _cairo_boilerplate_register_all (void);\n\n')

test/make-cairo-test-constructors.py

@@ -15,12 +15,12 @@ if __name__=='__main__':
     match_test_name = re.compile(r'^CAIRO_TEST.*\((.*),.*')
 
     for fname in args.input:
-        with open(fname, 'r') as f:
+        with open(fname, 'r', encoding='utf-8') as f:
             for l in f.readlines():
                 if match_test_line.match(l):
                     test_names.append(match_test_name.match(l).group(1))
 
-    with open(args.output, 'w') as f:
+    with open(args.output, 'w', encoding='utf-8') as f:
         f.write('/* WARNING: Autogenerated file - see %s! */\n\n' % sys.argv[0])
         f.write('#include "cairo-test-private.h"\n\n')
         f.write('void _cairo_test_runner_register_tests (void);\n\n')

version.py

@@ -14,7 +14,7 @@ if __name__ == '__main__':
     version_minor = None
     version_micro = None
 
-    f = open(os.path.join(srcroot, 'src', 'cairo-version.h'), 'r')
+    f = open(os.path.join(srcroot, 'src', 'cairo-version.h'), 'r', encoding='utf-8')
     for line in f:
         if line.startswith('#define CAIRO_VERSION_MAJOR '):
             version_major = line[28:].strip()