Fix flake8 issues (#3834)

* Fix flake8 whitespace reports

$ flake8 | grep -E '(E203|E221|E226|E303|W291|W293)'
./mesonbuild/coredata.py:337:5: E303 too many blank lines (2)

* Fix flake8 'variable assigned value but unused' reports

$ flake8 | grep -E F841
./mesonbuild/modules/gnome.py:922:9: F841 local variable 'target_name' is assigned to but never used

* Fix flake8 'imported but unused' reports

$ flake8 | grep F401
./mesonbuild/compilers/__init__.py:128:1: F401 '.c.ArmclangCCompiler' imported but unused
./mesonbuild/compilers/__init__.py:138:1: F401 '.cpp.ArmclangCPPCompiler' imported but unused
./mesonbuild/modules/__init__.py:4:1: F401 '..mlog' imported but unused

PR #3717 imports ARMCLANG compilers in __init__, but does not add them to
__all__, so they are not re-exported by the compilers package like
everything else.

* More details about flake8 in Contributing.md

Mention that Sider runs flake8
Suggest seting flake8 as a pre-commit hook

docs/markdown/Contributing.md

@@ -167,6 +167,8 @@ Continuous integration systems currently used:
   allows `[skip ci]` anywhere in the commit messages.
 - [AppVeyor](https://www.appveyor.com/docs/how-to/filtering-commits/#skip-directive-in-commit-message)
   requires `[skip ci]` or `[skip appveyor]` in the commit title.
+- [Sider](https://sider.review)
+  runs Flake8 (see below)
 
 ## Documentation
 
@@ -208,6 +210,13 @@ $ cd meson
 $ flake8
 ```
 
+To run it automatically before committing:
+
+```console
+$ flake8 --install-hook=git
+$ git config --bool flake8.strict true
+```
+
 ## C/C++ coding style
 
 Meson has a bunch of test code in several languages. The rules for

mesonbuild/compilers/__init__.py

@@ -46,6 +46,8 @@ __all__ = [
 
     'ArmCCompiler',
     'ArmCPPCompiler',
+    'ArmclangCCompiler',
+    'ArmclangCPPCompiler',
     'CCompiler',
     'ClangCCompiler',
     'ClangCompiler',

mesonbuild/coredata.py

@@ -333,7 +333,6 @@ class CoreData:
                     'Default project to execute in Visual Studio',
                     '')
 
-
     def get_builtin_option(self, optname):
         if optname in self.builtins:
             return self.builtins[optname].value

mesonbuild/modules/__init__.py

@@ -1,7 +1,6 @@
 import os
 
 from .. import build
-from .. import mlog
 
 
 class ExtensionModule:

mesonbuild/modules/gnome.py

@@ -919,7 +919,6 @@ This will become a hard error in the future.''')
             raise MesonException('gdbus_codegen takes at most two arguments, name and xml file.')
         namebase = args[0]
         xml_files = args[1:]
-        target_name = namebase + '-gdbus'
         cmd = [self.interpreter.find_program_impl('gdbus-codegen')]
         extra_args = mesonlib.stringlistify(kwargs.pop('extra_args', []))
         cmd += extra_args