add support for windows filepaths in dataset module

[timmysilv]

Context:
Windows uses \ as a path separator. Unfortunately, the datasets module matches local paths to S3 bucket paths and doesn't expect local paths to use anything other than /.

Description of the Change:
Convert windows paths to unix-like paths before sending requests to S3.

Benefits:
The dataset module will work as expected on Windows

Possible Drawbacks:
N/A (well.. encouraging people to continue development on Windows is a grey area but I digress 😄 )

Related GitHub Issues:
N/A

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #3504 (2e93d52) into master (d2ba09f) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3504      +/-   ##
==========================================
- Coverage   99.86%   99.86%   -0.01%     
==========================================
  Files         309      309              
  Lines       27432    27429       -3     
==========================================
- Hits        27396    27393       -3     
  Misses         36       36              

Impacted Files	Coverage Δ
pennylane/data/data_manager.py	100.00% <100.00%> (ø)
pennylane/data/dataset.py	100.00% <100.00%> (ø)

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[DSGuala]

A quick test on windows worked. I think the issue is solved.