Make JUnitXml.from* methods return only JUnitXml instances

[EnricoMi]

For the sake of consistency, JUnitXml.fromfile, JUnitXml.fromstring and JUnitXml.fromroot should always return a JUnitXml instance. Currently, when the root element of the XML document is a <TestSuite> tag, it would return a TestSuite instance. This causes confusion: #110, #111, #140.

This fact affects behavior of iterating the instance (as given in the examples), it iterates TestCase instances, not TestSuite instances:

from junitparser import JUnitXml

xml = JUnitXml.fromfile('/path/to/junit.xml')
for suite in xml:
    # handle suites
    for case in suite:
        # handle cases

https://github.com/weiwei/junitparser?tab=readme-ov-file#read-and-manipulate-existing-junitxunit-xml-files

Note: This is a breaking change.

Fixes #110
Fixes #111
Fixes #140

Based on #143, rebase after that has been merged.

[github-actions]

Test Results

  4 files  ±0    4 suites  ±0   1s ⏱️ ±0s
102 tests +1  102 ✅ +1  0 💤 ±0  0 ❌ ±0 
408 runs  +4  399 ✅ +4  9 💤 ±0  0 ❌ ±0 

Results for commit 5cbf10d. ± Comparison against base commit 16c54d7.

♻️ This comment has been updated with latest results.