diff --git a/appveyor.yml b/appveyor.yml
index c9db941..23bf2da 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -12,8 +12,8 @@ environment:
 
 install:
   - cmd: git submodule update --init --recursive
-  - cmd: SET PATH=%PATH%;C:\Python27\Scripts
-  - cmd: pip install -U https://github.com/platformio/platformio/archive/develop.zip
+  - cmd: SET PATH=%PATH%;C:\Python36\Scripts
+  - cmd: pip3 install -U https://github.com/platformio/platformio/archive/develop.zip
   - cmd: platformio platform install file://.
 
 test_script:
diff --git a/builder/frameworks/arduino.py b/builder/frameworks/arduino.py
index f981924..44fe6fd 100644
--- a/builder/frameworks/arduino.py
+++ b/builder/frameworks/arduino.py
@@ -22,6 +22,7 @@
 http://arduino.cc/en/Reference/HomePage
 """
 
+from io import open
 from os import listdir
 from os.path import isdir, isfile, join
 
@@ -270,14 +271,14 @@
             continue
         content = None
         content_changed = False
-        with open(file_path) as fp:
+        with open(file_path, encoding="latin-1") as fp:
             content = fp.read()
             if '#include "../' in content:
                 content_changed = True
                 content = content.replace('#include "../', '#include "')
         if not content_changed:
             continue
-        with open(file_path, "w") as fp:
+        with open(file_path, "w", encoding="latin-1") as fp:
             fp.write(content)
 else:
     env.Prepend(LIBPATH=[join(FRAMEWORK_DIR, "cores", "teensy3")])
diff --git a/platform.json b/platform.json
index d0ea780..12e8d4d 100644
--- a/platform.json
+++ b/platform.json
@@ -12,7 +12,7 @@
     "type": "git",
     "url": "https://github.com/platformio/platform-teensy.git"
   },
-  "version": "4.2.0",
+  "version": "4.2.1",
   "packageRepositories": [
     "https://dl.bintray.com/platformio/dl-packages/manifest.json",
     "http://dl.platformio.org/packages/manifest.json"