Fix Py3 failures when installing MSVC dependencies

python/servo/bootstrap.py

@@ -271,7 +271,8 @@ def check_cmake(version):
         cmake_path = find_executable("cmake")
         if cmake_path:
             cmake = subprocess.Popen([cmake_path, "--version"], stdout=PIPE)
-            cmake_version = cmake.stdout.read().splitlines()[0].replace("cmake version ", "")
+            cmake_version_output = six.ensure_str(cmake.stdout.read()).splitlines()[0]
+            cmake_version = cmake_version_output.replace("cmake version ", "")
             if LooseVersion(cmake_version) >= LooseVersion(version):
                 return True
         return False

python/servo/command_base.py

@@ -216,7 +216,7 @@ def is_linux():
 def append_to_path_env(string, env, name):
     variable = ""
     if name in env:
-        variable = env[name]
+        variable = six.ensure_str(env[name])
         if len(variable) > 0:
             variable += os.pathsep
     variable += string