fix deprecation warnings in regexes

Running .gitlab-ci/run_testcases.sh -m 'not pmaports_upstream_compat' in pmaports.git on alpine:latest shows the following warnings for the pmbootstrap code:

/tmp/pmbootstrap/pmb/install/_install.py:173
  /tmp/pmbootstrap/pmb/install/_install.py:173: DeprecationWarning: invalid escape sequence \/
    pmb.helpers.run.root(args, ["sed", "-i", "/\/mnt\/pmbootstrap-packages/d",

/tmp/pmbootstrap/pmb/install/_install.py:284
  /tmp/pmbootstrap/pmb/install/_install.py:284: DeprecationWarning: invalid escape sequence \.
    regex = ("s/^127\.0\.0\.1.*/127.0.0.1\t" + re.escape(hostname) +

/tmp/pmbootstrap/pmb/config/init.py:360
  /tmp/pmbootstrap/pmb/config/init.py:360: DeprecationWarning: invalid escape sequence \w
    validation_regex="^([-.+\w]+)(,[-.+\w]+)*$")

Example: https://gitlab.com/postmarketOS/pmaports/-/jobs/249897818

Related: https://gitlab.com/postmarketOS/pmaports/merge_requests/490#note_190903075

Let's fix these regexes.