Peter M. Groen / oletools

(1) Cannot open a temp file for writing from 2 python processes
    simultaneously
(2) Need proper environment setup for Popen (thanks to metatoaster's
    reply on stackoverflow https://stackoverflow.com/a/72845540/4405656)
(3) Need different os-separator, so replace hard-coded '/' with
    os.path.join or os.sep
(4) Minor cleanup of imports & whitespace in test_issue_*.py

There are still 2 tests that should be adapted to Windows, but those are
lower-prio in my eyes.

Also, unittest run on my windows VM creates many resource warnings due to
non-closed file descriptors. Should address that some (future) day.

To avoid triggering antivirus engines, we can encrypt
files with a default password and decrypt them before
testing.

Fix some PEP8-related linter complaints.

Word does not accept files which magic is not fully
lowercase.

With this change we replace the relative import.

Also use with-statement to open test file, so file is always closed after
reading (python3 unittest issued a warning).