Permission denied error in pip installing pyopenssl

Question

As I'm trying to force reinstall pyopenssl, I'm running into the following error when it tries to install pycparser. I am doing this on Windows 7 64-bit with Python 2.7 using the following command:

pip install pyopenssl --ignore-installed

Here is the traceback:

Exception:
Traceback (most recent call last):
    File "C:\Python27\ArcGIS10.4\lib\site-packages\pip-8.1.2-py2.7.egg\pip\basecommand.py", line 215, in main
        status = self.run(options, args)
    File "C:\Python27\ArcGIS10.4\lib\site-packages\pip-8.1.2-py2.7.egg\pip\commands\install.py", line 317, in run
        prefix=options.prefix_path,
    File "C:\Python27\ArcGIS10.4\lib\site-packages\pip-8.1.2-py2.7.egg\pip\req\req_set.py", line 742, in install
        **kwargs
    File "C:\Python27\ArcGIS10.4\lib\site-packages\pip-8.1.2-py2.7.egg\pip\req\req_install.py", line 831, in install
        self.move_wheel_files(self.source_dir, root=root, prefix=prefix)
    File "C:\Python27\ArcGIS10.4\lib\site-packages\pip-8.1.2-py2.7.egg\pip\req\req_install.py", line 1032, in move_wheel_files
        isolated=self.isolated,
    File "C:\Python27\ArcGIS10.4\lib\site-packages\pip-8.1.2-py2.7.egg\pip\wheel.py", line 346, in move_wheel_files
        clobber(source, lib_dir, True)
    File "C:\Python27\ArcGIS10.4\lib\site-packages\pip-8.1.2-py2.7.egg\pip\wheel.py", line 324, in clobber
        shutil.copyfile(srcfile, destfile)
    File "C:\Python27\ArcGIS10.4\lib\shutil.py", line 83, in copyfile
        with open(dst, 'wb') as fdst:
IOError: [Errno 13] Permission denied: 'C:\\Python27\\ArcGIS10.4\\Lib\\site-packages\\_cffi_backend.pyd'

Here is what I have noticed:

• It only happens with _cffi_backend.pyd - other packages install and uninstall fine.
• I can pip uninstall cffi and re-install it alright, but whenever it is present, the _cffi_backend.pyd permission error presents. This means that any installations calling cffi (for example pyopenssl) will fail.

Here is what I've tried:

1. Uninstalling and reinstalling cffi: no effect
2. Successfully used icalcs to set _cffi_backend.pyd to full access, everyone: no effect
3. Calling pip with python -m pip install pyopenssl --ignore-installed: no effect
4. Closed all applications and examined running processes before installing: no effect
5. Reboot. Voodoo for the most part, but just to be thorough: no effect