[Windows|Unix] Use double quote compatible both for windows and linux
diff --git a/tools/_venv_common.py b/tools/_venv_common.py
index ecd438f94c4..5408427734a 100755
--- a/tools/_venv_common.py
+++ b/tools/_venv_common.py
@@ -156,7 +156,7 @@ def main(venv_name, venv_args, args):
     new_environ['PATH'] = os.pathsep.join([get_venv_bin_path(venv_name), new_environ['PATH']])
     subprocess_with_print('python {0}'.format('./letsencrypt-auto-source/pieces/pipstrap.py'),
                           env=new_environ, shell=True)
-    subprocess_with_print("python -m pip install --upgrade 'setuptools>=30.3'",
+    subprocess_with_print('python -m pip install --upgrade "setuptools>=30.3"',
                           env=new_environ, shell=True)
     subprocess_with_print('python {0} {1}'.format('./tools/pip_install.py', ' '.join(args)),
                           env=new_environ, shell=True)

File `_venv_common.py` uses single quotes to ask `pip` to install `setuptools>=30.3`. Using single quotes to enclose a string is not supported on Windows shell (Batch). This PR replaces theses single quotes by double quotes supported both on Windows and Linux.