Explicit environ flag for importing eventlet.__version__ without ignoring import errors

Allows to read package version without installing dependencies.
```
import eventlet
print(eventlet.__version__)  # ok
eventlet.sleep()  # error
```

bin/release

@@ -13,7 +13,7 @@ version_next=
 
 main() {
 	branch="${1-$(git symbolic-ref --short HEAD)}"
-	version="$(python -c 'import eventlet; print(eventlet.__version__)')"
+	version="$(EVENTLET_IMPORT_VERSION_ONLY=1 python -c 'import eventlet; print(eventlet.__version__)')"
 	printf "\nbranch: %s eventlet.__version__: '%s'\n" $branch $version >&2
 	if [[ "$branch" != "master" ]]; then
 		echo "Must be on master" >&2
@@ -44,7 +44,7 @@ main() {
 		confirm "Continue still? [yN] " || exit 1
 	fi
 
-	if confirm "Build documentation (website)? [Yn] " >&2; then
+	if confirm "Build documentation (website)? [Yn] "; then
 		bin/build-website.bash || exit 1
 		git checkout "$branch"
 	fi

eventlet/__init__.py

@@ -1,7 +1,12 @@
+import os
+
+
 version_info = (0, 20, 0)
 __version__ = '.'.join(map(str, version_info))
-
+# This is to make Debian packaging easier, it ignores import
-try:
+# errors of greenlet so that the packager can still at least
+# access the version.  Also this makes easy_install a little quieter
+if os.environ.get('EVENTLET_IMPORT_VERSION_ONLY') != '1':
     from eventlet import greenthread
     from eventlet import greenpool
     from eventlet import queue
@@ -39,11 +44,3 @@ try:
     TimeoutError = timeout.Timeout
     exc_after = greenthread.exc_after
     call_after_global = greenthread.call_after_global
-except ImportError as e:
-    # This is to make Debian packaging easier, it ignores import
-    # errors of greenlet so that the packager can still at least
-    # access the version.  Also this makes easy_install a little quieter
-    if 'greenlet' not in str(e):
-        # any other exception should be printed
-        import traceback
-        traceback.print_exc()

setup.py

@@ -1,42 +1,47 @@
 #!/usr/bin/env python
-from setuptools import find_packages, setup
+import os
-from eventlet import __version__
+import setuptools
-from os import path
 
 
-setup(
+os.environ.setdefault('EVENTLET_IMPORT_VERSION_ONLY', '1')
+import eventlet
+
+setuptools.setup(
     name='eventlet',
-    version=__version__,
+    version=eventlet.__version__,
     description='Highly concurrent networking library',
     author='Linden Lab',
     author_email='eventletdev@lists.secondlife.com',
     url='http://eventlet.net',
-    packages=find_packages(exclude=['benchmarks', 'tests', 'tests.*']),
+    packages=setuptools.find_packages(exclude=['benchmarks', 'tests', 'tests.*']),
     install_requires=(
         'enum-compat',
         'greenlet >= 0.3',
     ),
     zip_safe=False,
     long_description=open(
-        path.join(
+        os.path.join(
-            path.dirname(__file__),
+            os.path.dirname(__file__),
             'README.rst'
         )
     ).read(),
     test_suite='nose.collector',
     classifiers=[
+        "Development Status :: 4 - Beta",
+        "Intended Audience :: Developers",
         "License :: OSI Approved :: MIT License",
-        "Programming Language :: Python",
         "Operating System :: MacOS :: MacOS X",
-        "Operating System :: POSIX",
         "Operating System :: Microsoft :: Windows",
+        "Operating System :: POSIX",
+        "Programming Language :: Python :: 2",
         "Programming Language :: Python :: 2.6",
         "Programming Language :: Python :: 2.7",
+        "Programming Language :: Python :: 3",
         "Programming Language :: Python :: 3.3",
         "Programming Language :: Python :: 3.4",
+        "Programming Language :: Python :: 3.5",
+        "Programming Language :: Python",
         "Topic :: Internet",
         "Topic :: Software Development :: Libraries :: Python Modules",
-        "Intended Audience :: Developers",
-        "Development Status :: 4 - Beta",
     ]
 )