executor: fix compatibility issue w/ py2 on windows

Python 2 running on Windows, causes thread leaks which
leads console io write errors when using pastel. This
works around the issue by limiting to a single worker
in these environments.

poetry/installation/executor.py

@@ -12,6 +12,8 @@ from subprocess import CalledProcessError
 from poetry.core.packages.file_dependency import FileDependency
 from poetry.core.packages.utils.link import Link
 from poetry.io.null_io import NullIO
+from poetry.utils._compat import PY2
+from poetry.utils._compat import WINDOWS
 from poetry.utils._compat import OrderedDict
 from poetry.utils._compat import Path
 from poetry.utils._compat import cpu_count
@@ -39,7 +41,7 @@ class Executor(object):
         self._chef = Chef(config, self._env)
         self._chooser = Chooser(pool, self._env)
 
-        if parallel:
+        if parallel and not (PY2 and WINDOWS):
             # This should be directly handled by ThreadPoolExecutor
             # however, on some systems the number of CPUs cannot be determined
             # (it raises a NotImplementedError), so, in this case, we assume