python3{5,6}: Reintroduce bytecode determinism

Originally introduced in 8970a9c and 1531b5e, these patches were lost in
efbe87f when the CPython version files were merged, likely due to
CPython 3.7 not needing them anymore. These patches should remain
in-tree until CPython 3.5 and 3.6 support is dropped completely.

Found with the diffoscope ( https://diffoscope.org/ ).

pkgs/development/interpreters/python/cpython/3.5/force_bytecode_determinism.patch

@@ -0,0 +1,17 @@
+--- a/Lib/py_compile.py
++++ b/Lib/py_compile.py
+@@ -139,3 +139,4 @@
+     source_stats = loader.path_stats(file)
++    source_mtime = 1 if 'DETERMINISTIC_BUILD' in os.environ else source_stats['mtime']
+     bytecode = importlib._bootstrap_external._code_to_bytecode(
+-            code, source_stats['mtime'], source_stats['size'])
++            code, source_mtime, source_stats['size'])
+--- a/Lib/importlib/_bootstrap_external.py
++++ b/Lib/importlib/_bootstrap_external.py
+@@ -485,5 +485,5 @@
+     if source_stats is not None:
+         try:
+-            source_mtime = int(source_stats['mtime'])
++            source_mtime = 1
+         except KeyError:
+             pass