If one or more iptables rule contains a comment with a non-unicode character, an UnicodeDecodeError would be raised.

* Add support to run inside Linux namespace

**Motivation:**
In a specific use case, we use sshuttle to provide access to private
networks from multiple sites to a specific host. The sites may contain
networks that overlap each other, so each site is accessed inside a
different namespace that provides process-level network isolation and
prevents network overlap.

**Objective:**
This commit just adds a convenient way of spawning multiple sshuttle
instances inside different namespaces from a single process, by passing
the namespace's name though the variable --namespace. The result is the
same as calling `ip netns exec $NAMESPACE sshuttle ...`

* Add the argument --namespace-pid

The argument '--namespace-pid' allows sshuttle to attach to the same net
namespace used by a running process.

* PEP-8 compliance

* Add comment

* Make --namespace and --namespace-pid mutually exclusive.

* Prevent UnicodeDecodeError parsing iptables rule with comments

If one or more iptables rule contains a comment with a non-unicode character, an UnicodeDecodeError would be raised.

Bumps [flake8](https://github.com/pycqa/flake8) from 7.1.1 to 7.1.2.
- [Commits](PyCQA/flake8@7.1.1...7.1.2)

---
updated-dependencies:
- dependency-name: flake8
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

* Update pip installation instructions to work
  without setup.py.

* Remove duplication of installation instructions
  in two places.