ci: add .flake8 linting to ci scripts and tests

Currently the ci scripts and tests don't have any linting checks. Add
.flake8 linting to start adding some consistency to the scripts. Ignore
most of the existing errors until they can be addressed on an individual
basis.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32602>

.gitlab-ci/.flake8

@@ -0,0 +1,33 @@
+[flake8]
+exclude = .venv*,
+
+# PEP 8 Style Guide limits line length to 79 characters
+max-line-length = 167
+
+ignore =
+    # continuation line under-indented for hanging indent
+    E121
+    # continuation line over-indented for hanging indent
+    E126,
+    # continuation line under-indented for visual indent
+    E128,
+    # whitespace before ':'
+    E203,
+    # missing whitespace around arithmetic operator
+    E226,
+    # missing whitespace after ','
+    E231,
+     # expected 2 blank lines, found 1
+    E302,
+    # too many blank lines
+    E303,
+    # imported but unused
+    F401,
+    # f-string is missing placeholders
+    F541,
+    # local variable assigned to but never used
+    F841,
+    # line break before binary operator
+    W503,
+    #  line break after binary operator
+    W504,