Update SOURCE_DATE_EPOCH documentation re. datetime.datetime.fromtimestamp....

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### Python

### Python >= 3.x

```

import os

import time

import datetime

build_date = datetime.datetime.utcfromtimestamp(

    int(os.environ.get('SOURCE_DATE_EPOCH', time.time()))

build_date = datetime.datetime.fromtimestamp(

    int(os.environ.get('SOURCE_DATE_EPOCH', time.time())),

    tz=datetime.timezone.utc,

)

```

or with less imports, rendering to a string:

… or with fewer imports and rendering to a string:

```

import os

import time

date_str = time.strftime(

    "%Y-%m-%d",

    time.gmtime(int(os.environ.get('SOURCE_DATE_EPOCH', time.time())))

)

```

### Python >= 2.x

If you still require Python 2.x support, you will need to use the non-recommended [`datetime.utcfromtimestamp`](https://docs.python.org/3.8/library/datetime.html#datetime.datetime.utcfromtimestamp) method ([more info](https://blog.ganssle.io/articles/2019/11/utcnow.html)):

```

import os

import time

import datetime

build_date = datetime.datetime.utcfromtimestamp(

    int(os.environ.get('SOURCE_DATE_EPOCH', time.time()))

)

```

### Bash / POSIX shell

For GNU systems: