MAINT: Drop support for Python 3.6 and 3.7

Summary

For #125 (closed). Remove support for Python versions not longer present in the two most recent Ubuntu-LTS versions

Focus

Nothing added, just increased minimal Python version. Have a look at the TODO in connection.py.

Test Environment

Usual pipeline is sufficent; verify that tox doesn't even attempt unsupported Python versions.

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src/caosdb/connection/connection.py

@@ -179,6 +179,8 @@ class _DefaultCaosDBServerConnection(CaosDBServerConnection):
             try:
                 ssl_version = ssl.PROTOCOL_TLS
             except AttributeError:
+                # TODO(tfitschen): This can be removed since we require Python
+                # >= 3.8 anyway, can't it?
                 # deprecated since Python 3.6, but necessary until there.
                 try:
                     ssl_version = ssl.PROTOCOL_TLSv1_2