chore: fix/improve flake8 configuration

Fix & improve a bit the configuration for flake8, so it can be run
without issues:
- drop all the old ignores, no more needed (all the issues where fixed)
- drop the excludes for paths that are either do not exist anymore, or
  that it is not a problem to scan (e.g. "docs")
- add the explicit filename patterns so "*.py.in" files are scanned too;
  adding ".py" is needed since "filename" replaces the default paths
- set the same line length as used by black
- add a couple of known ignores related to the style formatted by black
- ignore longer lines in a couple of files; they are there already, and
  fixing them can be done gradually
- exclude also the default meson build directory

Signed-off-by: Pino Toscano <ptoscano@redhat.com>

.flake8

@@ -1,3 +1,19 @@
 [flake8]
-ignore = E501,E126,E127,E128,E722
-exclude = bin,docs,include,lib,lib64,.git
+filename =
+    *.py,
+    *.py.in,
+# same limit as black
+max-line-length = 88
+ignore =
+    # E203 whitespace before ':'
+    # result of black-formatted code
+    E203,
+    # W503: line break before binary operator
+    W503
+per-file-ignores =
+    # some lines are longer than 88
+    src/insights_client/__init__.py: E501
+    integration-tests/test_unregister.py: E501
+extend-exclude =
+    # default build directory
+    build/,