Add option to allow relaxing the required match of python versions

[jmcarcell]

BEGINRELEASENOTES

• CMAKE: Add option PODIO_RELAX_PYVER to allow relaxing the required match of python version with the one that ROOT has been built with to only check major and minor versions

ENDRELEASENOTES

find_package will let everything go through but a later check will only allow mismatches in the patch version and throw and error if there is a mismatch in the major or minor versions. Does it make sense to have it ON by default since it's only the patch version? The error message looks like this:

CMake Error at CMakeLists.txt:102 (message):
  There is a mismatch between the major and minor versions in Python (found
  3.10) and ROOT, compiled with Python 3.9

in red, and stops cmake.

Also a few changes to lower case to be more consistent across the file

Closes #366

[andresailer]

Please no gratuitous changes like these

[jmcarcell]

Do I revert it? Doesn't it help to use the preferred cmake style? Unless we have one but this file has both lower and upper case

[tmadlener]

Yes, please revert for this PR. We can fix the inconsistencies in a different PR that only fixes that. Otherwise we will have commits that are mixing format / style changes and functional changes, which makes it hard to analyze the history.

[andresailer]

It still had mixed style after the couple changes. Yes, it would be better to use one style, but that should be CI enforced, and then we can use the git features to ignore them when using git blame.

[jmcarcell]

Ok those changes are now removed