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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• Reference related issues
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• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
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src/caosdb/connection/connection.py

@@ -176,16 +176,7 @@ class _DefaultCaosDBServerConnection(CaosDBServerConnection):
         if "ssl_version" in config and config["cacert"] is not None:
             ssl_version = getattr(ssl, config["ssl_version"])
         else:
-            try:
-                ssl_version = ssl.PROTOCOL_TLS
-            except AttributeError:
-                # deprecated since Python 3.6, but necessary until there.
-                try:
-                    ssl_version = ssl.PROTOCOL_TLSv1_2
-                except AttributeError:
-                    print("It seems as if your Python version does not support current encryption"
-                          "standards (such as TLS > 1.0), please upgrade to at least Python 3.4.")
-                    sys.exit(1)
+            ssl_version = ssl.PROTOCOL_TLS
         context = ssl.SSLContext(ssl_version)
         context.verify_mode = ssl.CERT_REQUIRED