fix failing tests on new tox >= 4.0 update

tox.ini

@@ -28,43 +28,43 @@ deps =
 
 
 # Earliest supported Scrapy version.
-[testenv:pinned-scrapy-2.0]
+[testenv:pinned-scrapy-2x0]
 basepython=python3.7
 deps =
     {[pinned]deps}
     scrapy==2.0.1
 
 # Scrapy version introducing Response.ip_address.
-[testenv:pinned-scrapy-2.1]
+[testenv:pinned-scrapy-2x1]
 basepython=python3.7
 deps =
     {[pinned]deps}
     scrapy==2.1.0
 
 # Latest Scrapy version since 2.0.1 not requiring to install the reactor early.
-[testenv:pinned-scrapy-2.3]
+[testenv:pinned-scrapy-2x3]
 basepython=python3.7
 deps =
     {[pinned]deps}
     scrapy==2.3.0
 
 # First Scrapy version requiring to install the reactor early.
-[testenv:pinned-scrapy-2.4]
+[testenv:pinned-scrapy-2x4]
 basepython=python3.7
 deps =
     {[pinned]deps}
     scrapy==2.4.0
 
 # Scrapy version introducing Response.protocol.
-[testenv:pinned-scrapy-2.5]
+[testenv:pinned-scrapy-2x5]
 basepython=python3.7
 deps =
     {[pinned]deps}
     scrapy==2.5.0
 
 # First Scrapy version since 2.4.0 where installing the reactor earlier is not
 # necessary.
-[testenv:pinned-scrapy-2.6]
+[testenv:pinned-scrapy-2x6]
 basepython=python3.7
 deps =
     {[pinned]deps}