bpo-29708: Setting SOURCE_DATE_EPOCH forces hash-based .pyc files (GH-5200)

bpo-29708: Setting SOURCE_DATE_EPOCH forces hash-based .pyc files (GH-5200)

To support reproducible builds, the setting of of SOURCE_DATE_EPOCH triggers the py_compile module -- and by extension, compileall -- to forcibly compile with hash-based .pyc files. This eliminates the possibility of timestamp-based .pyc files which vary between builds.