[2.7] bpo-30694: Upgrade Modules/expat/ to libexpat 2.2.1 (#2300)

[vstinner]

New file: Modules/expat/siphash.h.
(cherry picked from commit 5ff7132)

[vstinner]

I tested manually the change on Linux, test_pyexpat pass:

git clean -fdx  # remove all untracked files
./configure -C --with-pydebug CFLAGS=-O0 --enable-unicode=ucs4 && make && ./python -m test -v test_pyexpat 

[vstinner]

Oh, the compilation of expat 2.2.1 on Windows using Visual Studio 2008 fails because of missing stdint.h header :-(

c:\projects\cpython\modules\expat\siphash.h(79): fatal error C1083: Cannot open include file: 'stdint.h': No such file or directory [C:\projects\cpython\PCbuild\_elementtree.vcxproj]

https://ci.appveyor.com/project/python/cpython/build/2.7.13+.3582

[the-knights-who-say-ni]

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[vstinner]

Oh, the compilation of expat 2.2.1 on Windows using Visual Studio 2008 fails because of missing stdint.h header :-(

So, I wrote a patch, and added it for my PR. Then I wanted to send it upstream, but... upstream, they already fixed the issue in master! libexpat/libexpat@c74d380

New try: I cherry-picked this commit on top of expat 2.2.1.

[vstinner]

I tested the latest version of this PR on Windows with VS 2008 on a fresh Git repository (I ran "git clean -fdx" to remove all untracked files): the compilation succeed and test_pyexpat pass!

[vstinner]

@the-knights-who-say-ni: "Unfortunately we couldn't find an account corresponding to your GitHub username on bugs.python.org (b.p.o) to verify you have signed the CLA (...)"

Oh, I kept the original author of the commit in my cherry-pick: "Sebastian Pipping [email protected]", but as expected, this libexpat contributor isn't related to Python :-/ I took the ownership of this commit, sorry Sebastian! Anyway, I already replaced the Git ownership of all files in Modules/expat/ :-)