python: Open the template as text, with an explicit encoding

python: Open the template as text, with an explicit encoding

In commit bd27203f4d808763ac24ac94eb677cacf3e7cb99 we changed this to
open in binary mode, to then explicitly decode the lines with the right
encoding.

Unfortunately, that broke the build on Windows, where the template file
can have '\r\n' as line terminators: opening in binary mode would keep
those terminators and break the regexp.

We need to go back to text mode, where the "universal newlines" mode
takes care of this.

However, to fix the initial issue, let's specify the encoding explicitly
when opening the file, and make sure it is open in text mode, so we only
get unicode strings.

Reviewed-by: Jose Fonseca <jfonseca@vmware>