Modify mssql-cli bash wrapper to look for and prefer python3

On Ubuntu 20.04, the following install via pip does not work:

```bash
pip3 install --user mssqlcli
```
There are instructions to use a deb package for 18.04, but that's not an option. Related issues: #505, #482

In the bash wrapper to run this, I noticed python, and not python3, is assumed as the executable at https://github.com/dbcli/mssql-cli/blob/53a7a8a35193b1d2c8c8d85a2cdd7a4c3e3e7409/mssql-cli#L16

Some distributions (at least Debian/Ubuntu) facilitate the transition from python2 to python3 by having python point to python2 and python3 be a separate binary.

Instead of:

```bash
python -m mssqlcli.main "$@"
```

The following way of using command to check and match might help:

```bash
python_exe=$(command -v python3 || command -v python)
$python_exe -m mssqlcli.main "$@"
```

Then absolute path should be nicely resolved, e.g. /usr/bin/python3 for systems that have a binary found in the PATH.