Fix import ssl may fail under some Python installs

It's only required for certain proxy configurations, so we
don't want it to raise ImportError while the user imports
our library.

telethon/network/connection/connection.py

@@ -1,9 +1,13 @@
 import abc
 import asyncio
 import socket
-import ssl as ssl_mod
 import sys
 
+try:
+    import ssl as ssl_mod
+except ImportError:
+    ssl_mod = None
+
 from ...errors import InvalidChecksumError
 from ... import helpers
 
@@ -68,6 +72,12 @@ class Connection(abc.ABC):
                 loop=self._loop
             )
             if ssl:
+                if ssl_mod is None:
+                    raise RuntimeError(
+                        'Cannot use proxy that requires SSL'
+                        'without the SSL module being available'
+                    )
+
                 s.settimeout(timeout)
                 s = ssl_mod.wrap_socket(
                     s,