Make aiosmtpd dependency optional

Move the aiosmtpd dependency to "smtp" extra, and handle the lack of it
gracefully in tests.  The aiosmtpd package is no longer maintained
and had major bugs that lead to it being queued for removal from Gentoo.
FWICS all packages in Gentoo that are using pytest-localserver are only
using the HTTP support.

Author: mgorny

setup.py

@@ -35,8 +35,12 @@ def run(self):
     python_requires='>=3.5',
     install_requires=[
         'werkzeug>=0.10',
-        'aiosmtpd'
     ],
+    extras_require={
+        'smtp': [
+            'aiosmtpd',
+        ],
+    },
     cmdclass={'test': PyTest},
     tests_require=[
         'pytest>=2.0.0',

tests/test_smtp.py

@@ -1,11 +1,15 @@
+import pytest
 import smtplib
 
 try:  # python 3
     from email.mime.text import MIMEText
 except ImportError:  # python 2?
     from email.MIMEText import MIMEText
 
-from pytest_localserver import plugin, smtp
+from pytest_localserver import plugin
+
+
+smtp = pytest.importorskip('pytest_localserver.smtp')
 
 
 def send_plain_email(to, from_, subject, txt, server=('localhost', 25)):

tox.ini

@@ -24,6 +24,8 @@ deps =
     pytest-cov
     six
     requests
+extras =
+    smtp
 commands =
   py.test -v \
     --junitxml=junit-{envname}.xml \