[python linting] tweak options

Summary:
There's no need to specify --select when we're picking all the options.

It turns out that autopep8 with W50x options is a bit buggy, so disable that for now.
hhatto/autopep8#518

Test Plan:
    arc lint --everything

(tried on flake8 3.5.0, 3.6.0, and 3.7.9 latest)

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5219

.arclint

@@ -17,16 +17,15 @@
             "version": ">=1.3.4",
             "include": "(\\.py$)",
             "flags": [
-                "--select=E,W",
-                "--aggressive"
+                "--aggressive",
+                "--ignore=W503,W504"
             ]
         },
         "flake8": {
             "type": "flake8",
             "version": ">=3.0",
             "include": "(\\.py$)",
             "flags": [
-                "--select=E,W,F",
                 "--ignore=E501,E704,W503,W504"
             ]
         },