demandimport: ignore `lzma` module for demandimport
This makes importing the module fail if the `_lzma` module is not present.
This makes e.g. tarfile correctly recognize if LZMA support is not present. It
changes the exception
File "/usr/lib/python3.6/tarfile.py", line 1694, in xzopen
fileobj = lzma.LZMAFile(fileobj or name, mode, preset=preset)
AttributeError: module 'lzma' has no attribute 'LZMAFile'
to the more correct exception
File "/usr/lib/python3.6/tarfile.py", line 1692, in xzopen
raise CompressionError("lzma module is not available")
tarfile.CompressionError: lzma module is not available
Also, it prevents that the error "abort: No module named '_lzma'!" is shown when
a development warning is to be shown. The reason why that happened is that for
showing the warning, we get information about the stack frames from the inspect
module, which accesses the `__file__` attribute of all modules in `sys.modules`
to build some cache, causing all modules (including `lzma`) to be imported.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).