Fix re string type

[jhunkeler]

• Replaces string literal with a raw string
• https://docs.python.org/3/library/re.html#module-re states: [..] any invalid escape sequences in Python’s usage of the backslash in string literals now generate a SyntaxWarning and in the future this will become a SyntaxError. The solution is to use Python's raw string notation for regular expression patterns.

This PR addresses ...

Under Python 3.10:

DeprecationWarning: invalid escape sequence '\.'
    _VERSION_RE = re.compile(f"^[0-9]+\.[0-9]+\.[0-9]+$")

Under 3.11:

    _VERSION_RE = re.compile(f"^[0-9]+\.[0-9]+\.[0-9]+$")
                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
SyntaxError: invalid escape sequence '\.'

Under 3.12:

SyntaxWarning: invalid escape sequence '\.'
    _VERSION_RE = re.compile(f"^[0-9]+\.[0-9]+\.[0-9]+$")

Tasks

• update or add relevant tests
• update relevant docstrings and / or docs/ page
• Does this PR change any API used downstream? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)

news fragment change types...

• changes/<PR#>.hst.rst: HST reference files
• changes/<PR#>.jwst.rst: JWST reference files
• changes/<PR#>.roman.rst: Roman reference files
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.testing.rst: change to tests or test automation
• changes/<PR#>.general.rst: infrastructure or miscellaneous change