Add tox env with editable installs

src/tox.ini

@@ -21,7 +21,7 @@
 ## in a virtual environment.
 ##
 [tox]
-envlist = doctest, coverage, startuptime, pycodestyle-minimal, relint, codespell
+envlist = py{38,39}, doctest, coverage, startuptime, pycodestyle-minimal, relint, codespell
 # When adding environments above, also update the delegations in SAGE_ROOT/tox.ini
 skipsdist = true
 
@@ -34,6 +34,21 @@ setenv =
  SAGE={toxinidir}/../sage
 envdir={toxworkdir}/sagedirect
 
+[testenv]
+# Use editable install of main sagemath package
+usedevelop=True
+# Use bash hack as long as tox doesn't support multiple install commands: https://github.com/tox-dev/tox/issues/715
+allowlist_externals=bash
+install_command = bash -c '. {toxinidir}/bin/sage-env-config && . {toxinidir}/bin/sage-env && python -m pip install --editable=file:///{toxinidir}/../pkgs/sage-conf && python -m pip install --find-links "$SAGE_SPKG_WHEELS" "$@"' bash {opts} {packages}
+deps = -rrequirements.txt
+passenv =
+ HOME
+setenv =
+ SAGE_NUM_THREADS=12
+ SAGE_SPKG_WHEELS={toxinidir}/../local/var/lib/sage/venv-python3.8/var/lib/sage/wheels
+commands = 
+ python bin/sage-runtests {posargs:--all}
+
 [testenv:doctest]
 description =
  run the Sage doctester (same as "sage -t")