Allow old version of Python 2.7 to use TLS

The modern implementation of ssl using SSLContext appeared in
Python 2.7.9. Previous versions can use encrypted connections but
cannot precisely configure it.

Fixes #301

doc/src/concepts.rst

@@ -89,6 +89,13 @@ When constructing a custom context it is usually best to start with
 the default context, created by the ``ssl`` module, and modify it to
 suit your needs.
 
+.. warning::
+
+  Users of Python 2.7.0 - 2.7.8 can use TLS but cannot configure
+  the settings via an ``ssl.SSLContext``. These Python versions are
+  also not capable of proper certification verification. It is highly
+  encouraged to upgrade to a more recent version of Python.
+
 The following example shows how to to disable certification
 verification and certificate host name checks if required.
 
@@ -112,18 +119,6 @@ The above examples show some of the most common TLS parameter
 customisations but there are many other tweaks are possible. Consult
 the Python 3 :py:mod:`ssl` package documentation for further options.
 
-Old pyOpenSSL Versions
-+++++++++++++++++++++++
-
-IMAPClient's TLS functionality will not behaviour correctly if an
-out-of-date version of pyOpenSSL is used. On some systems
-(particularly OS X) the system installed version of pyOpenSSL will
-take precedence over any user installed version. Use of virtualenvs is
-strongly encouraged to work around this.
-
-IMAPClient checks the installed pyOpenSSL version at import time and
-will fail early if an old pyOpenSSL version is found.
-
 Using gevent with IMAPClient
 ++++++++++++++++++++++++++++
 Some extra monkey patching is required so that the gevent_ package can

imapclient/tls.py

@@ -13,6 +13,19 @@
 
 
 def wrap_socket(sock, ssl_context, host):
+
+    if not hasattr(ssl, 'create_default_context'):
+        # Python 2.7.0 - 2.7.8 do not have the concept of ssl contexts.
+        # Thus we have to use the less flexible and legacy way of wrapping the
+        # socket
+        if ssl_context is not None:
+            raise RuntimeError(
+                "Cannot precisely configure the SSL connection, upgrade to "
+                "Python >= 2.7.9 to fine tune the settings."
+            )
+
+        return ssl.wrap_socket(sock)
+
     if ssl_context is None:
         ssl_context = ssl.create_default_context(purpose=ssl.Purpose.CLIENT_AUTH)
 

imapclient/version.py

@@ -21,5 +21,3 @@ def _imapclient_version_string(vinfo):
 
 author = 'Menno Finlay-Smits'
 author_email = 'inbox@menno.io'
-
-min_pyopenssl_version = '0.15.1'