tests: fix pytest-nbval, tidy versions

tox.ini

@@ -25,12 +25,13 @@ PLATFORM=
 
 [core]
 deps=
+    py3{4,5,6}: pytest<7
     pytest
     pytest-cov
     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@named_cells#egg=nbval
+    py3{6,7,8,9,10}: git+https://github.com/casperdcl/nbval.git@master#egg=nbval
     coverage
     coveralls
     codecov
@@ -48,15 +49,15 @@ deps=
     matplotlib
     numpy
     pandas
+    keras: keras
+    tf: tensorflow!=2.5.0
     py27-keras: keras<2.5
     py27-tf: protobuf<3.18
-    !py27-keras: keras
     py3{6,7,8,9,10}: rich
-    tf: tensorflow!=2.5.0
 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{0,1,2,3,4,5,6}: 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{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