Address tox issues (#6240)

* Force Python 3.8 in pinned tox envs

* Force werkzeug < 3 when using mitmproxy

tox.ini

@@ -16,6 +16,9 @@ deps =
     #mitmproxy >= 5.3.0; python_version >= '3.9' and implementation_name != 'pypy'
     # The tests hang with mitmproxy 8.0.0: https://github.com/scrapy/scrapy/issues/5454
     mitmproxy >= 4.0.4, < 8; python_version < '3.9' and implementation_name != 'pypy'
+    # https://github.com/pallets/werkzeug/pull/2768 breaks flask, required by
+    # mitmproxy.
+    werkzeug < 3; python_version < '3.9' and implementation_name != 'pypy'
 passenv =
     S3_TEST_FILE_URI
     AWS_ACCESS_KEY_ID
@@ -71,6 +74,7 @@ commands =
     twine check dist/*
 
 [pinned]
+basepython = python3.8
 deps =
     cryptography==36.0.0
     cssselect==0.9.1
@@ -97,7 +101,7 @@ commands =
     pytest --cov=scrapy --cov-report=xml --cov-report= {posargs:--durations=10 scrapy tests}
 
 [testenv:pinned]
-basepython = python3.8
+basepython = {[pinned]basepython}
 deps =
     {[pinned]deps}
     PyDispatcher==2.0.5
@@ -107,7 +111,7 @@ setenv =
 commands = {[pinned]commands}
 
 [testenv:windows-pinned]
-basepython = python3
+basepython = {[pinned]basepython}
 deps =
     {[pinned]deps}
     PyDispatcher==2.0.5
@@ -130,7 +134,7 @@ deps =
     Twisted[http2]
 
 [testenv:extra-deps-pinned]
-basepython = python3.8
+basepython = {[pinned]basepython}
 deps =
     {[pinned]deps}
     boto3==1.20.0
@@ -147,6 +151,7 @@ commands =
     {[testenv]commands} --reactor=asyncio
 
 [testenv:asyncio-pinned]
+basepython = {[pinned]basepython}
 deps = {[testenv:pinned]deps}
 commands = {[pinned]commands} --reactor=asyncio
 install_command = {[pinned]install_command}
@@ -159,7 +164,7 @@ commands =
     pytest {posargs:--durations=10 docs scrapy tests}
 
 [testenv:pypy3-pinned]
-basepython = {[testenv:pypy3]basepython}
+basepython = pypy3.8
 deps =
     {[pinned]deps}
     PyPyDispatcher==2.1.0
@@ -212,7 +217,7 @@ commands =
     pytest --cov=scrapy --cov-report=xml --cov-report= {posargs:tests -k s3}
 
 [testenv:botocore-pinned]
-basepython = python3.8
+basepython = {[pinned]basepython}
 deps =
     {[pinned]deps}
     botocore==1.4.87