Specified encoding in setup.py calls of open() (#3137)

* Specified encoding in setup.py calls of open()

* Fix for Python2

Co-authored-by: Karsten Bock <karsten.bock@dlr.de>

setup.py

@@ -11,6 +11,7 @@ import string
 import subprocess
 import sys
 import tempfile
+import io
 
 import setuptools.command.sdist
 
@@ -70,7 +71,7 @@ exec(code, loc)
 version = loc["__version__"]
 
 # Verify that the version matches the one in C++
-with open("include/pybind11/detail/common.h") as f:
+with io.open("include/pybind11/detail/common.h", encoding="utf8") as f:
     matches = dict(VERSION_REGEX.findall(f.read()))
 cpp_version = "{MAJOR}.{MINOR}.{PATCH}".format(**matches)
 if version != cpp_version: