tests: drop py3.6 nbval

tox.ini

@@ -31,7 +31,7 @@ deps=
     pytest-timeout
     py3{7,8,9,10}: pytest-asyncio
     py3{6,7,8,9,10}: ipywidgets
-    py3{6,7,8,9,10}: git+https://github.com/casperdcl/nbval.git@master#egg=nbval
+    py3{7,8,9,10}: git+https://github.com/casperdcl/nbval.git@master#egg=nbval
     coverage
     coveralls
     codecov
@@ -55,12 +55,11 @@ deps=
     py27-tf: protobuf<3.18
     py3{6,7,8,9,10}: rich
 commands=
-    py36: pytest --cov=tqdm --cov-report= tests_notebook.ipynb --nbval --nbval-current-env -W=ignore --nbval-sanitize-with=setup.cfg -o addopts= -v --tb=short -rxs -W=error --durations=0 --durations-min=0.1
     py3{7,8,9,10}: pytest --cov=tqdm --cov-report= tests_notebook.ipynb --nbval --nbval-current-env -W=ignore --nbval-sanitize-with=setup.cfg
-    py3{4,5,6}: pytest --cov=tqdm --cov-report=xml --cov-report=term --cov-append -k "not perf" -o addopts= -v --tb=short -rxs -W=error --durations=0 --durations-min=0.1
     py3{7,8,9,10}: pytest --cov=tqdm --cov-report=xml --cov-report=term --cov-append -k "not perf"
     {[core]commands}
 allowlist_externals=codacy
+    py3{4,5,6}: pytest --cov=tqdm --cov-report=xml --cov-report=term -k "not perf" -o addopts= -v --tb=short -rxs -W=error --durations=0 --durations-min=0.1
 
 [testenv:py{27,py2}{,-tf}{,-keras}]
 commands=