Remove install_command from tox.ini

tox.ini

@@ -87,10 +87,6 @@ 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}
 setenv =
     _SCRAPY_PINNED=true
 
@@ -99,8 +95,6 @@ deps =
     {[pinned]deps}
     lxml==3.5.0
     PyDispatcher==2.0.5
-install_command =
-    {[pinned]install_command}
 setenv =
     {[pinned]setenv}
 
@@ -112,8 +106,6 @@ 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}
 setenv =
     {[pinned]setenv}
 
@@ -122,10 +114,6 @@ deps =
     {[testenv]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]
 commands =
@@ -133,8 +121,6 @@ commands =
 
 [testenv:asyncio-pinned]
 deps = {[testenv:pinned]deps}
-install_command =
-    {[pinned]install_command}
 commands = {[testenv:asyncio]commands}
 setenv =
     {[pinned]setenv}
@@ -150,8 +136,6 @@ deps =
     {[pinned]deps}
     lxml==4.0.0
     PyPyDispatcher==2.1.0
-install_command =
-    {[pinned]install_command}
 commands = {[testenv:pypy3]commands}
 setenv =
     {[pinned]setenv}