I can't reproduce with Python 2.7 locally, and the error makes no
sense (because we're writing the banner to a *brand bloody new* file,
guaranteed not to exist). So this is a shot in the dark.

See
https://ci.appveyor.com/project/denik/gevent/build/1.0.568/job/p44icqoi91a72e08.

Failed to rename 'c:\users\appveyor\appdata\local\temp\1\tmp_j5icn\gevent\corecext.pyx.tmp.2832' to 'c:\users\appveyor\appdata\local\temp\1\tmp_j5icn\gevent\corecext.pyx
Exception in thread Thread-1:
Traceback (most recent call last):
  File "C:\Python27\lib\threading.py", line 801, in __bootstrap_inner
    self.run()
  File "util\cythonpp.py", line 31, in run
    self.value = target(*args)
  File "util\cythonpp.py", line 432, in _run_cython_on_file
    atomic_write(unique_pyx_filename, py_banner + value)
  File "util\cythonpp.py", line 902, in atomic_write
    os.rename(tmpname, filename)
WindowsError: [Error 32] The process cannot access the file because it is being used by another process