Add a portable env for py27 in tox.ini,

update python3 envs and PYTHONWARNINGS, restrict deps for reset and stats.

tox.ini

@@ -1,11 +1,11 @@
 [tox]
-envlist = reset,py{27,37,38},stats
+envlist = reset,py{27,27-portable,36,38,39},stats
 skip_missing_interpreters = true
 
 [testenv]
 setenv =
     BITMESSAGE_HOME = {envtmpdir}
-    PYTHONWARNINGS = all
+    PYTHONWARNINGS = default
 deps = -rrequirements.txt
 commands =
     python checkdeps.py
@@ -18,10 +18,16 @@ deps =
     -r docs/requirements.txt
 commands = python setup.py build_sphinx
 
+[testenv:py27-portable]
+skip_install = true
+commands = python pybitmessage/bitmessagemain.py -t
+
 [testenv:reset]
+deps = coverage
 commands = coverage erase
 
 [testenv:stats]
+deps = coverage
 commands =
     coverage report
     coverage xml