mtools should use python3 in shebangs

[p-mongo]

Current version of mtools (specifically, mlaunch) requires python 3 and does not work on python 2. However, it retains the /usr/bin/env python shebang lines.

This is causing problems on systems where the default python is python 2. In these cases mtools installs but with an exception as follows:

[2019/12/30 16:11:47.683] + mlaunch --dir /data/mci/d1c82ce78a6313e78ba96cd703415531/src/../drivers-tools/.evergreen/orchestration/db --binarypath /data/mci/d1c82ce78a6313e78ba96cd703415531/src/../drivers-tools/.evergreen/orchestration/mdb/mongodb-linux-x86_64-enterprise-ubuntu1404-4.0.9/bin --single --sslMode requireSSL --sslPEMKeyFile spec/support/certificates/server-second-level-bundle.pem --sslCAFile spec/support/certificates/ca.crt --sslClientCertificate spec/support/certificates/client.pem
[2019/12/30 16:11:47.694] Traceback (most recent call last):
[2019/12/30 16:11:47.694]   File "/data/mci/d1c82ce78a6313e78ba96cd703415531/src/../drivers-tools/.evergreen/orchestration/python/bin/mlaunch", line 5, in 
[2019/12/30 16:11:47.694]     from mtools.mlaunch.mlaunch import main
[2019/12/30 16:11:47.694]   File "/data/mci/d1c82ce78a6313e78ba96cd703415531/drivers-tools/.evergreen/orchestration/python/mtools/mlaunch/mlaunch.py", line 1541
[2019/12/30 16:11:47.694]     result.append(f'"{arg}"')
[2019/12/30 16:11:47.694]                            ^
[2019/12/30 16:11:47.694] SyntaxError: invalid syntax
[2019/12/30 16:11:47.696] Command failed: command encountered problem: error waiting on process 'c0de4964-271f-4412-91b7-ee8acaa83f4d': exit status 1

https://www.python.org/dev/peps/pep-0394/ discusses the behavior of the "python" command. According to my reading, this pep does not prescribe or recommend any particular behavior of this command.

It is my understanding that python3 is available if python 3 is installed. As far as I can recall, all python 3 installations I worked with provided a python3 binary. Therefore it should be safe to change shebang lines in mtools to specify python3 rather than python, which would make mtools work on systems that provide python 2 as the default python.

Expected behavior

mlaunch works

Actual/current behavior

mlaunch does not work because it is run with python 2 interpreter

Steps to reproduce the actual/current behavior

Use mlaunch on Evergreen

Environment

Software	Version
mtools	1.6.0
MongoDB server	any
Operating system	Linux