Changing os.path to posixpath to prevent issues

[novama]

Changing os.path to posixpath to prevent issues when using pyminio in Windows environments.

os.path uses the current system file system separator and adjust all its functions to that. MinIO is always a posix path system. When using Pyminio in a Windows system, the paths got undesirable changed, because os.path set them to comply with Windows file system format.

This update prevents that issue to happen.

• [X] All tests are green.

Related issue: https://github.com/IamTugy/pyminio/issues/36