Fix list of supported Python version

Signed-off-by: Pierre Fersing <[email protected]>

ChangeLog.txt

@@ -1,6 +1,8 @@
-v1.7.0 - 2023-xx-xx
+v2.0.0 - 2023-xx-xx
 ===================
 
+- **BREAKING** Drop support for Python 2.7, Python 3.5 and Python 3.6
+  Minimum tested version is Python 3.7
 - Add on_pre_connect() callback, which is called immediately before a
   connection attempt is made.
 

tox.ini

@@ -1,14 +1,14 @@
 [tox]
-envlist = py{27,35,36,37,38,39}
-
-[testenv:py27]
-setenv = EXCLUDE = --exclude=./.*,./examples/loop_asyncio.py,*/MQTTV5.py,*/MQTTV311.py
+envlist = py{37,38,39}
 
 [testenv]
 whitelist_externals = echo make
 deps =
     -rrequirements.txt
     flake8
+allowlist_externals =
+    echo
+    make
 commands =
     # $EXCLUDE is defined above in testenv:py27 as a workaround for Python 2
     # which does not support asyncio and type hints
@@ -17,3 +17,10 @@ commands =
     make -C test test
     # TODO (cclauss) Fix up all these undefined names
     flake8 . --count --exit-zero --select=F821 --show-source --statistics
+
+# On older Python, flake8 version 4 fail with:
+# RecursionError: maximum recursion depth exceeded
+[testenv:py37]
+deps =
+    -rrequirements.txt
+    flake8<4