set_tls: incorrect default tls_version?

We are currently investigating an issue in our software, and came across this weird construction that might or might not be related to the problems we're experiencing.

In v1.6.1 the documentation says the following about the tls_version parameter:
> tls_version allows the version of the SSL/TLS protocol used to be
specified. **By default TLS v1.2 is used.** Previous versions are allowed
but not recommended due to possible security problems.

So... naturally what I would assume is that if I don't pass this parameter, it would default to TLSv1.2. However, in practice we're seeing different behaviour when passing this default explicitly, and not passing it at all.
This can be explained by the code that handles the setting of the default:
```
if tls_version is None:
tls_version = ssl.PROTOCOL_TLSv1_2
# If the python version supports it, use highest TLS version automatically
if hasattr(ssl, "PROTOCOL_TLS"):
tls_version = ssl.PROTOCOL_TLS
```
So indeed, first the default is set to `ssl.PROTOCOL_TLSv1_2`, but afterwards it is overridden again by `ssl.PROTOCOL_TLS`!
I think the documentation in its current state is very misleading, since it does not indicate this. Similar confusing statements were present in previous versions also.

A second problem with this default value that I noticed is that the comment says it will use the highest version automatically. However, I noticed that `ssl.PROTOCOL_TLS` resolves to int 2. This is the same as `PROTOCOL_SSLv23`, which is definitely not the highest version available.
Based on #653, I tried changing `ssl.PROTOCOL_TLS` to `ssl.PROTOCOL_TLS_CLIENT`. If I do that, the int resolves to int 16. I do not know with what enum this corresponds (couldn't find it. It's also not known to Python PDB), but at least it sounds like that makes more sense?

Is this a bug? Or is this working as intended? And if it's a bug, is it a bug in paho-mqtt, or in openssl-python?