Add optional srvhostname argument for managesieve client

[wildegnux]

Add optional srvhostname argument for managesieve client to support passing differnt server_hostname to ssllib context.

My use case is the following.
I'm using this library for a test that managesieve (dovecot pigeonhole) is working as expected with TLS for a hostname. The hostname resolves to two ip addresses and I want to test each ip address individually.
The certificate served by dovecot is valid for the hostname, not the IP addresses and thus if I try connecting without this patch I would get the following error:

sievelib.managesieve.Error: SSL error: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: IP address mismatch, certificate is not valid for '1.2.3.4'. (_ssl.c:1032)

passing the correct hostname instead of the ip to the sslib context fixes this.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.56%. Comparing base (69f644c) to head (0914161).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
sievelib/managesieve.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #126   +/-   ##
=======================================
  Coverage   87.55%   87.56%           
=======================================
  Files          10       10           
  Lines        2073     2074    +1     
=======================================
+ Hits         1815     1816    +1     
  Misses        258      258           

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[tonioo]

@wildegnux Thanks for this contribution