"Unable to find or execute python" python details set version=3 only works for python3.7

Nobody has claimed this yet.

in an Ubuntu18.04 container, I keep getting this error:
```
* What went wrong:
A problem occurred evaluating root project 'example-project'.
> Unable to find or execute python
```
I tried install python3.5, 3.6 and 3.8. but only python3.7 works.
I know the work around would be setting systemPythonInterpreter = file("/usr/bin/python3"). But it's not portable: works on ubuntu but doesn't work on my mac (which has different path: /usr/local/bin/python3)

I'd be appreciate if someone can look at the issue and point out why is it the case. I'm not familiar with kotlin so I got very impatient when I look at the code base..

I have reproduced this issue in example-project with this Dockerfile and build.gradle (the original build.gradle had many errors)
**Dockerfile:**
```
FROM ubuntu:18.04

RUN apt update && apt install python3-distutils python3 vim git openjdk-8-jdk gradle -y

RUN git clone https://github.com/linkedin/pygradle.git

COPY build.gradle /pygradle/examples/example-project/

CMD ["bash"]
```

**build.gradle**
```
buildscript {
repositories {
mavenLocal()
maven {
url "https://plugins.gradle.org/m2/"
}
mavenCentral()
jcenter()
}
dependencies{
classpath "gradle.plugin.com.linkedin.pygradle:pygradle-plugin:0.9.11"
}
}

apply plugin: "com.linkedin.python-sdist"
apply plugin: com.linkedin.gradle.python.plugin.WheelFirstPlugin

python {
details {
pythonVersion = '3'
}
forceVersion('pypi', 'flake8', '3.2.1')
forceVersion('pypi', 'six', '1.13.0')
forceVersion('pypi', 'pbr','1.10.0')
}

dependencies {
python 'pypi:requests:2.9.1'
test 'pypi:mock:2.0.0'
}

repositories {
pyGradlePyPi()
}
```

The reproduction is in examples/example-project/build.gradle and the supplied Dockerfile; start by running it with the listed Python 3.x installations and inspect the plugin's interpreter-selection behavior. Done means pythonVersion = '3' resolves the installed interpreter consistently without a hard-coded OS-specific path.

Written by the indexing model from the issue text.