Add basic SSL support

This is a partial support as the CA certificate is not validated. This
is so as this functionality is not implemented in all micropython ports.

For example, it isn't currently supported in the esp32 port although a
draft of the feature has been published:
micropython/micropython#5998

As the CA functionality is not there, we cannot use this parameter
do decide when to switch to SSL mode as done in the CPython version.
Instead we enable SSL when connecting to port 443 or 8443 which are
well known ports used for TLS communications.

One more thing is that this library relies on short reads to get data
from the socket as it always ask for the max buffer length when reading.
In micropython, the interface provided for SSL socket is only the one
of a stream which doesn't allow short reads. To go around this we change
the socket to non blocking which which has the side-effect of allowing
short reads. However we then need to do manual polling and timeout on
the socket which we do here.

Author: fgervais

blynklib_mp.py

@@ -5,6 +5,7 @@
 __version__ = '0.2.6'
 
 import usocket as socket
+import ussl as ssl
 import utime as time
 import ustruct as struct
 import uselect as select
@@ -125,6 +126,7 @@ def internal_msg(self, *args):
 class Connection(Protocol):
     SOCK_MAX_TIMEOUT = const(5)
     SOCK_TIMEOUT = 0.05
+    SOCK_SSL_TIMEOUT = const(1)
     EAGAIN = const(11)
     ETIMEDOUT = const(60)
     RETRIES_TX_DELAY = const(2)
@@ -164,15 +166,28 @@ def send(self, data):
             try:
                 retries -= 1
                 self._last_send_time = ticks_ms()
-                return self._socket.send(data)
+                try:
+                    bytes_written = self._socket.send(data)
+                except AttributeError:
+                    bytes_written = self._socket.write(data)
+                return bytes_written
             except (IOError, OSError):
                 sleep_ms(self.RETRIES_TX_DELAY)
 
     def receive(self, length, timeout):
         d_buff = b''
         try:
             self._set_socket_timeout(timeout)
-            d_buff += self._socket.recv(length)
+            try:
+                d_buff += self._socket.recv(length)
+            except AttributeError:
+                timeout = self.SOCK_SSL_TIMEOUT
+                while not d_buff and timeout > 0:
+                    ret = self._socket.read(length)
+                    if ret:
+                        d_buff += ret
+                    timeout -= self.SOCK_TIMEOUT
+                    time.sleep(self.SOCK_TIMEOUT)
             if len(d_buff) >= length:
                 d_buff = d_buff[:length]
             return d_buff
@@ -203,6 +218,13 @@ def _get_socket(self):
             self._socket = socket.socket()
             self._socket.connect(socket.getaddrinfo(self.server, self.port)[0][-1])
             self._set_socket_timeout(self.SOCK_TIMEOUT)
+            if self.port == 443 or self.port == 8443:
+                self.log('Using SSL socket...')
+                self._socket = ssl.wrap_socket(self._socket)
+                # Short reads are not supported in ssl mode. We work around
+                # this by setting the socket non blocking and doing manual
+                # polling/timeout.
+                self._socket.setblocking(False)
             self.log('Connected to server')
         except Exception as g_exc:
             raise BlynkError('Server connection failed: {}'.format(g_exc))