Behavior of `killpg` slightly differs in Mac and Linux. For some reason,
`killpg` returns error EPERM when sending a signal to a zomibie process.
And that is the reason of `test_process_management` failure on mac -
there's a small gap between killing child and and subreaper calls
`waitpid`.

Now pytest handles this exception properly.

Close https://git.picodata.io/picodata/picodata/picodata/-/issues/70

See also:

- Stackoverflow: Why would `killpg` return "not permitted" when ownership is correct?
  https://stackoverflow.com/questions/12521705/why-would-killpg-return-not-permitted-when-ownership-is-correct

- Linux `man 2 killpg`:
  https://linux.die.net/man/2/killpg#Notes

  > Notes
  >
  > There are various differences between the permission checking in
  > BSD-type systems and System V-type systems. See the POSIX rationale
  > for kill(). A difference not mentioned by POSIX concerns the return
  > value EPERM: BSD documents that no signal is sent and EPERM returned
  > when the permission check failed for **at least one** target process,
  > while POSIX documents EPERM only when the permission check failed for
  > **all** target processes.

- MacOS `man 2 killpg`:
  https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/killpg.2.html

  > [EPERM] The sending process is not the super-user and
  >         **one or more** of the target processes has an effective
  >         user ID different from that of the sending process.

- Linux `man 2 kill`:
  https://linux.die.net/man/2/kill

  >  EPERM  The process does not have permission to send the signal
  >         *to any* of the target processes.
  >

- Process states in Linux:
  https://kerneltalks.com/linux/process-states-in-linux/

- Reproduce killpg returning EPERM on MacOS:
  https://git.picodata.io/picodata/picodata/picodata/-/snippets/7

Waiting for a valid `leader_id` on a node isn't enough. It may already
have one, but still be a Learner. Instead, the fixture should wait until
the node is promoted to voter.

1. Review `Pipfile`:
  - Remove unused `filelock`;
  - Install `mypy` - static type checker for Python.
2. Add new command `pipenv run lint`.
3. Enable `mypy` in CI. Fix reported errors in `test_basics.py`.
4. Renew readme.

In `conftest.py`:

  - *Add* function `xdist_worker_number`. It converts `str(worker_id)` into `int`.
    It serves as a substitute for `session_data_mutex` for parallel test runs.

  - *Change* `normalize_net_box_result`. Replace a function with a decorator. Also,
    handle typical kinds of picodata responses. Extensively test it in
    `test_basics.py::test_eval/call_normalization`.

  - *Add* dataclass `RaftStatus`. It shouldn't be used outside `conftest.py`. It only makes
    assertions more brief in logs and understndable in code.

  - *Add* all raft stuff into the `Instance` class. This implies `raft_propose_eval`,
    `assert_raft_status` all `promote_or_fail` moved from `util.py`.

  - *Change* fixture `compile`. No need in extra logics since commit 59c31cb8.

  - *Preserve* fixture `binary_path`.

  - *Remove* fixtures `session_data_mutex` and `run_id`. Superseded with `xdist_worker_number`.

  - *Remove* fixtures `run_cluster` and `run_instance`. Superseded with `cluster.deploy(...)`.

  - *Remove* function `wait_tcp_port`. It's never enough to check raw socket. Superseded with
    `instance.wait_ready()`.

  - Give the instances clean names `i1, i2, ...`, and simple addresses `127.7.0.1:3301`.
    For the parallel test run use different IPs `127.7.N.1` etc.

In `test_basics.py`:

  - *Add* `test_xdist_worker_number`.

  - *Add* `test_call_normalization` and `test_eval_normalization`.

  - *Add* `test_process_management`. It's brand new, never implemented in luatest before.

  - *Rename* `test_single_instance_raft_eval` to `test_propose_eval` and extend it with additional
    assertion from `single_test.lua`.

  - *Remove* `test_instance`. A part of its logics is moved to `test_call/eval_normalization`. The
    other part is rewritten and extended in `test_process_management`.

  - *Remove* `test_cluster`. It was completely useless because of inappropriate synchronization and
    no valuable assertions.

  - *Remove* `test_propose_eval`. It wasn't that useful, but failed because of inappropriate
    synchronization.

In `test_couple.py`:

  - *Preserve* `test_follower_proposal` and `test_failover`. Just slightly refactor according to the
    new `conftest.py` API.

In `util.py` (completely removed):

  - *Remove* decorator `retry`. Needless.

  - *Remove* decorator `retry_on_network_errors`. Inappropriate predicate didn't catch Lua errors.

  - *Remove* everything related to raft. Move it into `conftest.py`.

Part of https://git.picodata.io/picodata/picodata/picodata/-/issues/59