Remove mitmproxy from pinned environments

tests/requirements-py3.txt

@@ -1,8 +1,6 @@
 # Tests requirements
 attrs
 dataclasses; python_version == '3.6'
-mitmproxy; python_version >= '3.7'
-mitmproxy >= 4.0.4, < 5; python_version >= '3.6' and python_version < '3.7'
 pyftpdlib
 # https://github.com/pytest-dev/pytest-twisted/issues/93
 pytest != 5.4, != 5.4.1

tests/test_proxy_connect.py

@@ -5,8 +5,6 @@ import re
 import sys
 from subprocess import Popen, PIPE
 from urllib.parse import urlsplit, urlunsplit
-from unittest import skipIf
-
 from testfixtures import LogCapture
 from twisted.internet import defer
 from twisted.trial.unittest import TestCase
@@ -57,13 +55,14 @@ def _wrong_credentials(proxy_url):
     return urlunsplit(bad_auth_proxy)
 
 
-@skipIf("pypy" in sys.executable,
-        "mitmproxy does not support PyPy")
-@skipIf(platform.system() == 'Windows' and sys.version_info < (3, 7),
-        "mitmproxy does not support Windows when running Python < 3.7")
 class ProxyConnectTestCase(TestCase):
 
     def setUp(self):
+        try:
+            import mitmproxy
+        except ImportError:
+            self.skipTest('mitmproxy is not installed')
+
         self.mockserver = MockServer()
         self.mockserver.__enter__()
         self._oldenv = os.environ.copy()

tox.ini

@@ -11,6 +11,10 @@ minversion = 1.7.0
 deps =
     -ctests/constraints.txt
     -rtests/requirements-py3.txt
+    # mitmproxy does not support PyPy
+    # mitmproxy does not support Windows when running Python < 3.7
+    mitmproxy; python_version >= '3.7' and implementation_name != 'pypy'
+    mitmproxy >= 4.0.4, < 5; python_version >= '3.6' and python_version < '3.7' and platform_system != 'Windows' and implementation_name != 'pypy'
     # Extras
     boto3>=1.13.0
     botocore>=1.4.87
@@ -26,7 +30,7 @@ download = true
 commands =
     py.test --cov=scrapy --cov-report= {posargs:--durations=10 docs scrapy tests}
 install_command =
-    pip install --use-feature=2020-resolver {opts} {packages} 
+    pip install --use-feature=2020-resolver {opts} {packages}
 
 [testenv:typing]
 basepython = python3
@@ -78,6 +82,10 @@ deps =
     w3lib==1.17.0
     zope.interface==4.1.3
     -rtests/requirements-py3.txt
+
+    # mitmproxy 4.0.4+ requires upgrading some of the pinned dependencies
+    # above, hence we do not install it in pinned environments at the moment
+
     # Extras
     botocore==1.4.87
     google-cloud-storage==1.29.0