Use the new pip resolver for Tox environments with pinned dependencies

tox.ini

@@ -88,12 +88,18 @@ deps =
     botocore==1.4.87
     google-cloud-storage==1.29.0
     Pillow==4.0.0
+install_command =
+    # --use-feature=2020-resolver is required, otherwise the latest verion of
+    # Twisted gets installed.
+    pip install --use-feature=2020-resolver {opts} {packages}
 
 [testenv:pinned]
 deps =
     {[pinned]deps}
     lxml==3.5.0
     PyDispatcher==2.0.5
+install_command =
+    {[pinned]install_command}
     
 [testenv:windows-pinned]
 basepython = python3
@@ -103,6 +109,8 @@ deps =
     # not need to build lxml from sources in a CI Windows job:
     lxml==3.8.0
     PyDispatcher==2.0.5
+install_command =
+    {[pinned]install_command}
 
 [testenv:extra-deps]
 deps =
@@ -110,6 +118,8 @@ deps =
     reppy
     robotexclusionrulesparser
 install_command =
+    # Test --use-feature=2020-resolver for the latest version of all
+    # dependencies.
     pip install --use-feature=2020-resolver {opts} {packages}
 
 [testenv:asyncio]
@@ -118,6 +128,8 @@ commands =
 
 [testenv:asyncio-pinned]
 deps = {[testenv:pinned]deps}
+install_command =
+    {[pinned]install_command}
 commands = {[testenv:asyncio]commands}
 
 [testenv:pypy3]
@@ -131,6 +143,8 @@ deps =
     {[pinned]deps}
     lxml==4.0.0
     PyPyDispatcher==2.1.0
+install_command =
+    {[pinned]install_command}
 commands = {[testenv:pypy3]commands}
 
 [docs]