Fix regression with distutils MANIFEST handing (#11104, #8688).

The changed behavior of sdist in 3.1 broke packaging for projects that
wanted to use a manually-maintained MANIFEST file (instead of having a
MANIFEST.in template and letting distutils generate the MANIFEST).

The fixes that were committed for #8688 (76643c286b9f by Tarek and
d54da9248ed9 by me) did not fix all issues exposed in the bug report,
and also added one problem: the MANIFEST file format gained comments,
but the read_manifest method was not updated to handle (i.e. ignore)
them.  This changeset should fix everything; the tests have been
expanded and I successfully tested the 2.7 version with Mercurial, which
suffered from this regression.

I have grouped the versionchanged directives for these bugs in one place
and added micro version numbers to help users know the quirks of the
exact version they’re using.

Initial report, thorough diagnosis and patch by John Dennis, further
work on the patch by Stephen Thorne, and a few edits and additions by
me.
diff --git a/Misc/ACKS b/Misc/ACKS
index db451d7..d345e54 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -215,6 +215,7 @@
 Vincent Delft
 Arnaud Delobelle
 Erik Demaine
+John Dennis
 Roger Dev
 Raghuram Devarakonda
 Caleb Deveraux
@@ -875,6 +876,7 @@
 Tobias Thelen
 James Thomas
 Robin Thomas
+Stephen Thorne
 Jeremy Thurgood
 Eric Tiedemann
 July Tikhonov