[1.5.x] Fixed #19922 - Typo in translation docs.

Thanks amoebob for the report.

Backport of 9ce1b6191b from master
[py3] Always fed hashlib with bytes.
[1.5.x] Fixed #16807 - Added a class-based views intro.

Thanks Preston Holmes for the text.

Backport of 2108941677 from master
[1.5.x] Fixed #19926 -- Fixed a link to code example in queries docs

Thanks Randy Salvo for the report.
Backport of b0ba21db07 from master.
[1.5.x] [py3] str.decode does not exist; str.encode was intended

Backport of fba6df19b5 from master
[1.5.x] Fixed #19937 - Typo in class-based views intro.

Backport of d009ffe436 from master
[1.5.x] Fixed #19960 -- Fixed sentence in contrib.auth signals docs

Thanks edd at slipszenko.net for the report.
Backport of 6d3b0c33dc from master.
[1.5.x] Fixed #19962 - Added a note about SESSION_EXPIRE_AT_BROWSER_CLOSE and browsers that persist sessions.

Thanks David Sanders.

Backport of fe5d9fe5fe from master
[1.5.x] Fixed #19933 -- Catched ValueError in get_system_username

Thanks lanre.hbs at gmail.com for the report.
[1.5.x] Fixed #20008 -- Removed trailing slash in Wikipedia link

Thanks senden9 at gmail.com for the report.
Backport of 63c52dcbcd from master.
[1.5.x] Fixed #19965 - Added a warning that the tutorial is written for Python 2.

Thanks itsallvoodoo for the patch.

Backport of e1bafdbffa from master.
[1.5.x] Fixed #20036 -- Improved GEOS version string parsing

Thanks chikiro.spam at gmail.com for the report.
Backport of 747f7d254 from master.
[1.5.x] Updated our six module to follow upstream changes.

This includes fixes for the java/jython detection and a new license header.

Thanks to Thomas Bartelmess for the report.

Backport of c5ce0e8a687ecf7fbc38fa85c5311a6320a246c6 from master.
[1.5.x] Updated bundled version of six.

Backport of e11ccc76d325b6bbbf101f510a91299f507f0745.
[1.5.x] Stopped using non-standard __globals__ and __code__ attributes.

Some alternative implementations don't have them.

Closes #19944.

Backport of 9d4a5b00f1977d8cddf8e241fa4bb8a9e819f188 from master.
[1.5.x] Minor docs fix for e11ccc76.

Backport of f3a6d74 from master.
[1.5.x] Fixed #20051 -- Specified where to define `TEMPLATE_DIRS`.
[1.5.x] Revert "Fixed #19895 -- Made second iteration over invalid queryset raise an exception too"

This reverts commit d1e87eb3baf75b1b6a0ada46a9b77f7e347cdb60.
This commit was the cause of a memory leak. See ticket for more details.
Thanks Anssi Kääriäinen for identifying the source of the bug.
[1.5.x] Fixed #20096 -- Added link to the Greek localflavor app

Backport of a9ee0e297 from master.
[1.5.x] Fixed #17935 - Clarified intro of topics/files.txt.

Thanks guettli for the suggestion.

Backport of f7795e968d from master