tox: stop testing on python < 3.8

No need to test on versions we don't support anymore

tox.ini

@@ -27,7 +27,7 @@ envlist =
     pypy3-test-instrumentation-aiohttp-client
 
     ; opentelemetry-instrumentation-aiohttp-server
-    py3{6,8,9,10,11}-test-instrumentation-aiohttp-server
+    py3{8,9,10,11}-test-instrumentation-aiohttp-server
     pypy3-test-instrumentation-aiohttp-server
 
     ; opentelemetry-instrumentation-aiopg
@@ -44,7 +44,7 @@ envlist =
     ; pypy3-test-instrumentation-botocore
 
     ; opentelemetry-instrumentation-boto3sqs
-    py3{6,8,9,10,11}-test-instrumentation-boto3sqs
+    py3{8,9,10,11}-test-instrumentation-boto3sqs
     ; FIXME: see https://github.com/open-telemetry/opentelemetry-python-contrib/issues/1736
     ; pypy3-test-instrumentation-boto3sqs
 
@@ -144,7 +144,7 @@ envlist =
     pypy3-test-exporter-richconsole
 
     ; opentelemetry-exporter-prometheus-remote-write
-    py3{6,8,9,10,11}-test-exporter-prometheus-remote-write
+    py3{8,9,10,11}-test-exporter-prometheus-remote-write
     pypy3-test-exporter-prometheus-remote-write
 
     ; opentelemetry-instrumentation-mysql
@@ -160,7 +160,7 @@ envlist =
     ; ext-psycopg2 intentionally excluded from pypy3
 
     ; opentelemetry-instrumentation-psycopg
-    py3{7,8,9,10,11}-test-instrumentation-psycopg
+    py3{8,9,10,11}-test-instrumentation-psycopg
     pypy3-test-instrumentation-psycopg
 
     ; opentelemetry-instrumentation-pymemcache
@@ -230,7 +230,7 @@ envlist =
     py3{8}-test-instrumentation-sklearn
 
     ; opentelemetry-instrumentation-system-metrics
-    py3{6,8,9,10,11}-test-instrumentation-system-metrics
+    py3{8,9,10,11}-test-instrumentation-system-metrics
     pypy3-test-instrumentation-system-metrics
 
     ; opentelemetry-instrumentation-threading