tox.ini: modernize and enable execution without preinstalling deps

tox.ini

@@ -3,31 +3,29 @@
 
 [tox]
 envlist = py{36,37,38,39,310}-{none,fuse2,fuse3}
-
+minversion = 3.2
+requires =
+	pkgconfig
+	cython
+	wheel
+	setuptools_scm
 [testenv]
+setenv =
+   fuse2: BORG_FUSE_IMPL=llfuse
+   fuse3: BORG_FUSE_IMPL=pyfuse3
 deps =
+    fuse2: llfuse
+    fuse3: pyfuse3
     -rrequirements.d/development.txt
 commands = py.test -v -n {env:XDISTN:1} -rs --cov=borg --cov-config=.coveragerc --benchmark-skip --pyargs {posargs:borg.testsuite}
 # fakeroot -u needs some env vars:
 passenv = *
 
-[testenv:py{36,37,38,39}-fuse2]
-setenv =
-    BORG_FUSE_IMPL=llfuse
-deps =
-    llfuse
-    {[testenv]deps}
-
-[testenv:py{36,37,38,39}-fuse3]
-setenv =
-    BORG_FUSE_IMPL=pyfuse3
-deps =
-    pyfuse3
-    {[testenv]deps}
 
 [testenv:flake8]
+skip_sdist=true
+skip_install=true
 changedir =
 deps =
     flake8
-    pkgconfig
 commands = flake8 src scripts conftest.py