flake8 C90 is selected but max-complexity is not set

```
flake8 --select "C90" aiosmtpd --isolated --max-complexity 10
aiosmtpd/docs/_exts/autoprogramm.py:70:1: C901 'scan_programs' is too complex (11)
aiosmtpd/docs/_exts/autoprogramm.py:204:5: C901 'AutoprogrammDirective.make_rst' is too complex (13)
aiosmtpd/main.py:170:1: C901 'parseargs' is too complex (13)
aiosmtpd/main.py:217:1: C901 'main' is too complex (12)
aiosmtpd/proxy_protocol.py:166:5: C901 'ProxyTLV.parse' is too complex (13)
aiosmtpd/proxy_protocol.py:343:1: C901 '_get_v1' is too complex (19)
aiosmtpd/proxy_protocol.py:427:1: C901 '_get_v2' is too complex (16)
aiosmtpd/smtp.py:314:5: C901 'SMTP.__init__' is too complex (18)
aiosmtpd/smtp.py:613:5: C901 'SMTP._handle_client' is too complex (33)
aiosmtpd/smtp.py:838:5: C901 'SMTP.smtp_EHLO' is too complex (13)
aiosmtpd/smtp.py:943:5: C901 'SMTP.smtp_AUTH' is too complex (16)
aiosmtpd/smtp.py:1269:5: C901 'SMTP.smtp_MAIL' is too complex (20)
aiosmtpd/smtp.py:1342:5: C901 'SMTP.smtp_RCPT' is too complex (13)
aiosmtpd/smtp.py:1408:5: C901 'SMTP.smtp_DATA' is too complex (28)
```