python,python3: add support for SOURCE_DATE_EPOCH var

See:
https://github.com/openwrt/packages/issues/5278

This should make Python & Python3 packages reproducible
when building.
In my local tests, I got the same sha256 for a sample
.pyc file, so likely this is the solution that should address
this.

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

lang/python/python/patches/012-add-support-source-date-epoch-pyc.patch

@@ -0,0 +1,10 @@
+--- a/Lib/py_compile.py
++++ b/Lib/py_compile.py
+@@ -108,6 +108,7 @@ def compile(file, cfile=None, dfile=None, doraise=False):
+             timestamp = long(os.fstat(f.fileno()).st_mtime)
+         except AttributeError:
+             timestamp = long(os.stat(file).st_mtime)
++        timestamp = long(os.getenv('SOURCE_DATE_EPOCH', timestamp))
+         codestring = f.read()
+     try:
+         codeobject = __builtin__.compile(codestring, dfile or file,'exec')