python,python3: honour the SOURCE_DATE_EPOCH for the build-info

The Modules/getbuildinfo.c allows the use of DATE and TIME
macros to be defined via CFLAGS.

These vars, control the build date & time when the
interpreter is opened, and can be read via the
`platform._sys_version()` function.

So, a conversion from SOURCE_DATE_EPOCH to DATE & TIME
is required at build-time.

This is especially needed for `platform._sys_version()`
to work.
The installation of pip seems to rely on this.

The logic has been adapted from:
 https://wiki.debian.org/ReproducibleBuilds/TimestampsProposal#Makefile

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

lang/python/python3/patches/013-getbuildinfo-date-time-source-date-epoch.patch

@@ -0,0 +1,28 @@
+--- a/Makefile.pre.in
++++ b/Makefile.pre.in
+@@ -725,6 +725,16 @@ regen-all: regen-opcode regen-opcode-tar
+ ############################################################################
+ # Special rules for object files
+ 
++DATE_FMT = %b %d %Y
++TIME_FMT = %H:%M:%S
++ifdef SOURCE_DATE_EPOCH
++	BUILD_DATE ?= $(shell date -u -d "@$(SOURCE_DATE_EPOCH)" "+$(DATE_FMT)"  2>/dev/null || date -u -r "$(SOURCE_DATE_EPOCH)" "+$(DATE_FMT)" 2>/dev/null || date -u "+$(DATE_FMT)")
++	BUILD_TIME ?= $(shell date -u -d "@$(SOURCE_DATE_EPOCH)" "+$(TIME_FMT)"  2>/dev/null || date -u -r "$(SOURCE_DATE_EPOCH)" "+$(TIME_FMT)" 2>/dev/null || date -u "+$(TIME_FMT)")
++else
++	BUILD_DATE ?= $(shell date "+$(DATE_FMT)")
++	BUILD_TIME ?= $(shell date "+$(TIME_FMT)")
++endif
++
+ Modules/getbuildinfo.o: $(PARSER_OBJS) \
+ 		$(OBJECT_OBJS) \
+ 		$(PYTHON_OBJS) \
+@@ -732,6 +742,8 @@ Modules/getbuildinfo.o: $(PARSER_OBJS) \
+ 		$(MODOBJS) \
+ 		$(srcdir)/Modules/getbuildinfo.c
+ 	$(CC) -c $(PY_CORE_CFLAGS) \
++	      -DDATE="\"$(BUILD_DATE)\"" \
++	      -DTIME="\"$(BUILD_TIME)\"" \
+ 	      -DGITVERSION="\"`LC_ALL=C $(GITVERSION)`\"" \
+ 	      -DGITTAG="\"`LC_ALL=C $(GITTAG)`\"" \
+ 	      -DGITBRANCH="\"`LC_ALL=C $(GITBRANCH)`\"" \