fix: SSLConnection is readable when it's accepting

[moben]

ftps broke with recent curl clients (exact version unknown), due to pyftpdlib closing the dtp connection during the handshake via DTPHandler.readable() as the DTPHandler was not supposed to be readable yet. But the SSLConnection is accepting in this state, so it should be readable as far as I understand.

Example error:

# curl -v -k --ssl-reqd ftp://*****:*****@n10:2021/v0000/foo
*   Trying 27.1.0.10:2021...
* Connected to n10 (27.1.0.10) port 2021
< 220 ftp ready.
> AUTH SSL
< 234 AUTH SSL successful.
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* TLSv1.3 (IN), TLS handshake, Server hello (2):
* TLSv1.3 (OUT), TLS change cipher, Change cipher spec (1):
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* TLSv1.3 (IN), TLS handshake, Server hello (2):
* TLSv1.3 (IN), TLS handshake, Encrypted Extensions (8):
* TLSv1.3 (IN), TLS handshake, Certificate (11):
* TLSv1.3 (IN), TLS handshake, CERT verify (15):
* TLSv1.3 (IN), TLS handshake, Finished (20):
* TLSv1.3 (OUT), TLS handshake, Finished (20):
* SSL connection using TLSv1.3 / TLS_AES_256_GCM_SHA384
* Server certificate:
* -----
*  SSL certificate verify result: self signed certificate (18), continuing anyway.
> USER *****
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
* old SSL session ID is stale, removing
< 331 Username ok, send password.
> PASS *****
< 230 Login successful.
> PBSZ 0
< 200 PBSZ=0 successful.
> PROT P
< 200 Protection set to Private
> PWD
< 257 "/" is the current directory.
* Entry path is '/'
> CWD v0000
* ftp_perform ends with SECONDARY: 0
< 250 "/v0000" is the current directory.
> EPSV
* Connect data stream passively
< 229 Entering extended passive mode (|||65330|).
* Connecting to 27.1.0.10 (27.1.0.10) port 65330
*   Trying 27.1.0.10:65330...
* Connected to n10 (27.1.0.10) port 2021
* SSL reusing session ID
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
> TYPE I
* Recv failure: Connection reset by peer
* OpenSSL SSL_connect: Connection reset by peer in connection to n10:2021 
* Failed EPSV attempt. Disabling EPSV
> PASV
< 200 Type set to: Binary.
* Bad PASV/EPSV response: 200
* Remembering we are in dir "v0000/"
* Connection #0 to host n10 left intact
curl: (13) Bad PASV/EPSV response: 200

Fixes: #604