GHA: use pip `requirements.txt` with pins, and more venv

GHA: use pip `requirements.txt` with pins, and more venv

- requirements.txt: shorten copyright headers.

- requirements.txt: pin packages to versions.

- GHA/windows: use `tests/requirements.txt`.
  Pick a `cryptography` package version that satifies both `impacket`
  and pytests dependencies.

- GHA/checksrc: move pip deps into a new `requirements.txt`.
  To make Dependabot detect and bump them.

- GHA/checksrc: replace apt packages for python test deps with pip
  install `tests/**/requirements.txt` to a venv.

- GHA/checksrc: use venv and drop `--break-system-packages`.

- GHA/linux: fix to actually activate venvs.
  Follow-up to 2638570241cb9e68240d7621f0213916334a4765 #15578

- GHA/linux: fixup (did not cause an issue)
  Follow-up to d75785c7dea214d12525beb659694d3fcc483731 #18660

- GHA: create venvs later, simplify commands.

- GHA: sync pip command-line options, e.g. drop progress-bar,
  everywhere.

Assisted-by: Dan Fandrich
Closes #18708