Add a python version parameter and raise a warning when it is not set

WHY

Python 3 has many breaking changes compared to python 2. This has an impact on our bug detection rules when some code pattern is a bug in python 3 but not in python 2.
For example, during the last sprint we we wanted to fix some False Negatives on python 3 code. In the end we postponed this fix because it would have raised False Positive bugs on python 2 code.
We also want to better support builtin types, but many of those builtins changed between python 2 and python 3.

One might say that we shouldn't care about python 2 because its support was stopped on the first of January this year, and surveys show that 9 out of 10 python developers were using python 3.
However other sources show that there is still quite a lot of python 2 code running in some places. For example if you look at download statistics for Django, which is the biggest python web framework, you can see that 20% of total downloads are for python 2 software (current time: 18th of March 2020).

On the long run we want to focus on python 3 code, but we want to also support python 2 developers.

WHAT

This ticket will:

• add a configuration parameter "sonar.python.version" where developers can write the python version they use.
• show a warning in SonarQube/SonarCloud when this parameter is not set:
  Your code is analyzed as compatible with python 2 and 3 by default. This will prevent the detection of issues specific to python 2 or python 3. You can get a more precise analysis by setting a python version in your configuration via the parameter "sonar.python.version".

By default the parameter will have no value and the analyzer will suppose that the code is compatible for both python 2 and python 3. Thus any issue specific to python 2 or python 3 will not be raised.

The "pythonVersion" parameter will be a string following a subset of "Version Specifiers" as defined in PEP 440. Accepted syntax will be:

python_version=[0-9]+|[0-9]+.[0-9]+
version_constraint=(>=|<=|>|<)python_version
version_specifier=version_constraint(,version_constraint)*

Examples:

# python 2 and 3
>= 2
# python 3
>= 3 
# python 2
<= 2.7
# or
< 3
# python 3.6 only
>= 3.6, < 3.7

We won't accept other operators described in PEP 440 (==, !=, ~=, ===) because >=, <=, < and > are enough for our purpose. We won't accept any version more precise than a minor version ("patch", "rc", "alpha", "beta", etc...).

For now we won't make any distinction between python minor versions 2 and 3 (ex: 3.6). This however can be useful later as some apis were broken between minor versions (ex: added and removed methods/classes/modules). By enabling developers to give a more precise version right away, we will be able to raise better issues later on. We can't raise a warning later saying that the version is not precise enough.

Documentation

The documentation for the parameter should be similar to the one for sonar.java.source.
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Handling Python Source Version

Python code is analyzed by default as compatible with python 2 and python 3. Some issues will be automatically silenced to avoid raising False Positives. In order to get a more precise analysis you can specify the python versions your code supports via the sonar.python.version parameter. Its syntax is a subset of "Version Specifiers" as defined in PEP 440. Only major and minor versions are accepted as well as operators >, <, <= and >=.

Examples:

# python 2 and 3
>= 2
# python 3
>= 3 
# python 2
<= 2.7
# or
< 3
# python 3.6 only
>= 3.6, < 3.7

The minor version is not used yet but specifying it now will improve future analysis. For example, if your code is only compatible with python 3.6+ we recommend to set the version to >= 3.6.