Run tests without asyncio support by default, add py35-asyncio and py38-asyncio envs.

pytest.ini

@@ -5,7 +5,6 @@ python_classes=
 addopts =
     --assert=plain
     --doctest-modules
-    --reactor=asyncio
     --ignore=docs/_ext
     --ignore=docs/conf.py
     --ignore=docs/news.rst

tox.ini

@@ -107,8 +107,14 @@ deps =
     reppy
     robotexclusionrulesparser
 
-[testenv:py38-no-asyncio]
+[testenv:py35-asyncio]
+basepython = python3.5
+deps = {[testenv]deps}
+commands =
+    py.test --cov=scrapy --cov-report= --reactor=asyncio {posargs:scrapy tests}
+
+[testenv:py38-asyncio]
 basepython = python3.8
 deps = {[testenv]deps}
 commands =
-    py.test --cov=scrapy --cov-report= --reactor=default {posargs:scrapy tests}
+    py.test --cov=scrapy --cov-report= --reactor=asyncio {posargs:scrapy tests}