Upgrade minimum dependencies for Python 3.6 support

tests/requirements-py3.txt

@@ -1,8 +1,7 @@
 # Tests requirements
 attrs
 dataclasses; python_version == '3.6'
-mitmproxy; python_version >= '3.6'
-mitmproxy<4.0.0; python_version < '3.6'
+mitmproxy >=4, <5  # The latest version does not support some pinned dependencies
 # https://github.com/pytest-dev/pytest-twisted/issues/93
 pytest != 5.4, != 5.4.1
 pytest-azurepipelines

tox.ini

@@ -13,7 +13,7 @@ deps =
     -rtests/requirements-py3.txt
     # Extras
     boto3>=1.13.0
-    botocore>=1.3.23
+    botocore>=1.4.87
     Pillow>=3.4.2
 passenv =
     S3_TEST_FILE_URI
@@ -76,7 +76,7 @@ deps =
     zope.interface==4.1.3
     -rtests/requirements-py3.txt
     # Extras
-    botocore==1.3.23
+    botocore==1.4.87
     google-cloud-storage==1.29.0
     Pillow==3.4.2