Merge tox environment configuration

tox.ini

 [tox]
 envlist=flake8,mypy,py3
 
-[testenv:py3]
+[testenv]
 deps =
   # the dependency below is needed for now as a workaround for
   #   https://github.com/pypa/pip/issues/6239
   swh.core[http] >= 0.0.75
   .[testing]
+  dev: ipdb
   pytest-cov
   pifpaf
 commands =
-  pifpaf run postgresql -- pytest --cov {envsitepackagesdir}/swh/deposit --cov-branch \
-      {envsitepackagesdir}/swh/deposit \
-      {posargs}
-
-[testenv:py3-dev]
-deps =
-  swh.core[http] >= 0.0.75
-  .[testing]
-  ipdb
-commands =
-  pytest {envsitepackagesdir}/swh/deposit/ {posargs}
+  pifpaf run postgresql -- \
+    pytest \
+  !dev: --cov {envsitepackagesdir}/swh/deposit --cov-branch \
+        {envsitepackagesdir}/swh/deposit \
+        {posargs}
 
 [testenv:flake8]
 skip_install = true