Add optional encoding parameter to load and dump functions

[Backist]

An optional encoding parameter has been added to the dump and load functions to provide more flexibility in specifying encoding, rather than always using the system default.

[Backist]

After several hours of struggling with my machine with a GIT Bash problem, I present the final PR including the encoding parameter to the dump and load functions. Although I had initially set utf-8 as the default for enconding, it seemed that some tests were using different encodings, so I considered leaving it at None and letting the user set the required encoding.

[Backist]

PS: Apologies for the number of failed or cancelled PRs. Several of them were not correct or had late commits due to the problem with my GIT Bash that I mentioned.

[Backist]

@bobfang1992 Awaiting review. Everything should now be in order.

[bobfang1992]

Thanks, will merge if all CI passes