Fixed test_get_file_extensions() on Python 3.

(removed dependency on dictionary iteration order)
Corrected Typos

Fixed a spelling error and removed a double `import`
compatibility with tests and older django versions (1.4.)
final docs update for 3.5.5
Merges from upstream
updated dev-version to 3.5.6
Manual import of upstream commit 1f6a5ac
final doc updates for release 3.5.6
updated dev-version to 3.5.7