Fix flake8 errors

[set-soft]

This patch fixes all flake8 errors. The changes are big and many errors are fixed. I hope no new errors are introduced, but I can't be sure. I tested:

• Passes the tests for Python 2.7, 3.5, 3.7 and 3.9.
• A simple test to fetch prices seems to work.
• The GUI starts, but dies in the same way that the current HEAD does.

Note that the following issues needed adjust that you should check:

• Missing apiKey argument for api_octopart.skus_to_mpns
• api_octopart.get_part_info used idx in some places. I beleive this is i (used in most places)
• api_octopart.get_part_info used dist_part_num without defining it. I copied the use from KitSpace API.
• dist_local_template.py used urlsplit/urlunsplit without definition.
• edas/eda_kicad.py defined eda_altium class instead of eda_kicad
• edas/tools.py could use field_code_last before definition. Defined as None by default.
• kicad_config.py used psutil.process_iter without import. I removed the code because it isn't used (get_running_processes).
• kicad_config.py fields_remove_entry and fields_add_entry uses name where values seems to be the correct variable.
• kicost() used an undefined variable x. Changed by f.
• kicost_config.py used winreg, but for Python 2 it seems to be _winreg, like kicost_gui.py and os_windows.py use.
• kicost_gui.py used libreoffice_executable locally, but then tried to access it as a global definition.
• In os_windows.py reg_enum_values I commented some invalid dead code.

Also note that:

• In kicost_config.py: create_os_contex_menu returns True/False, no exceptions. They are catched.
  • Many functions are inside try/except. But in case of fail they return False and no exception. A review is needed.
• except Exception: should avoid the need of keyboard/system exclusion.
  • I used it in many places, but we should replace by the expected exceptions