odf2xliff: Use a real with block
xml_extract: Use a real with block
misc: Drop context.py (fake with blocks)
misc: Drop contextlib, use stdlib contextlib instead
Imports: Add isort

- Add info for isort in development setup
- Add default config for isort behaviour
  - Ignore a number of files and folders
Imports: setup.py
Imports: sort translate/convert/
Imports: sort tests/
Imports: sort translate/
Imports: sort translate/lang
Imports: sort translate/search
Imports: sort translate/misc
Imports: sort translate/services
Imports: sort translate/tools

No sorting of test_* or commandline scripts
Imports: sort translate/tools/test_*
Imports: sort translate/tools commandline tools
Imports: sort translate/filters
Imports: sort translate/storage
Imports: sort translate/storage/test_*
Imports: sort translate/convert