tests: use PYTHONSAFEPATH (-P) to check mercurial lib path

setup: require TLS 1.2 support from the Python interpreter (BC) Before it was optional, and either 1.1 or 1.2 was sufficient. Now that the default minimum is 1.2, it needs to be present to work out of the box. The code here is more convoluted than the corresponding checks in `sslutil.py`, but I'm leaving it alone because it can all be simplified when py38 is dropped.

run-tests: refactor the HGPORT handling code to use a constant for the max val Simple enough, and then we don't need comments to cross-reference other areas of code that need to be kept in sync. Since the `_getreplacements()` list is now partially dynamically generated and needed to change anyway, also move the one line that was being appended separately to the list for some reason, for clarity.

rust: fix darwin build

rust: remove `atty` dependency It is fully replaced with the now stable `std::io::IsTerminal` trait. This was the last dependency flagged as a warning by `cargo audit`, aside from `cpython` which we know about all too well: the plan is to transition to PyO3 soon-ish.

rust: update `env_logger` dependency Aside from being more up-to-date in general, this makes its own dependency on the unmaintained and now useless `atty` disappear.

rust: update `clap` to the latest 4.x version This brings in more up-to-date dependencies, some bug fixes (none of which are relevant yet), and slightly improved compile times.

rust: fix `cargo doc` warnings It makes sense to keep our doc build happy, even if it is lacking.