venv: '!' in --prompt breaks activate.csh (csh history expansion)

Bug report

venv quotes substituted strings in its activation scripts with POSIX
shlex.quote() for every script except .ps1/.bat (gh-124651). That is
wrong for activate.csh: (t)csh performs history expansion on ! even
inside single quotes, so a --prompt (or path) containing ! followed by a
word character produces an activate.csh that fails to source.

Reproducer

$ python -m venv --prompt 'py!env' /tmp/env
$ csh -c 'source /tmp/env/bin/activate.csh'
env: Event not found.

activate.csh runs setenv VIRTUAL_ENV_PROMPT 'py!env'; csh history-expands
!env, the line errors, and activation aborts (VIRTUAL_ENV/prompt unset).
Both csh and tcsh are affected; bash/zsh activate and activate.fish
are not (POSIX-compatible single quotes).

Cause

Lib/venv/__init__.py:replace_variables() dispatches the quoting function by
script suffix (.ps1 -> quote_ps1, .bat -> quote_bat, else -> shlex.quote).
activate.csh falls into the POSIX branch, which does not account for csh
history expansion.

Scope

--prompt is the cleanest case: the venv path is clean, so source works and
only the substituted prompt breaks -- fully fixable in venv. A ! in the venv
directory path is also mis-quoted by venv internally (same root cause), but
note that activating such a venv additionally requires the user to escape the
! in their own source <path> command line, which is outside venv's control.

Fix

Add a quote_csh that escapes !, and dispatch on .csh -- mirroring the
existing quote_ps1/quote_bat handling. Verified under csh and tcsh:
!, quotes, $, backtick, ;, &&, spaces, parens round-trip. (A newline
cannot be represented in a csh single-quoted string; that remains out of scope
and is unchanged from current behavior.)

Not a security issue: csh errors out rather than executing anything.

Linked PRs

• gh-152687

Lib/venv/init.py の replace_variables() から始め、既存の quote_ps1 と quote_bat のディスパッチを比較します。! を含む --prompt で失敗を再現し、その後、生成された activate.csh を csh と tcsh で source します。プロンプトとその他の一覧にある特殊文字が、アクティベーションを失敗させることなくラウンドトリップすれば完了です。