Guard optional bz2/gzip/lzma imports so pooch imports without them (fixes #468)

[gaoflow]

Fixes #468

Problem

bz2, gzip and lzma are optional features of the Python standard
library — an interpreter can be built without them when the corresponding
bzip2/lzma/zlib development headers are missing (reported on a
self-compiled Python 3.13 and on a custom Python 3.9 on RHEL in #468).

pooch/processors.py imported all three at module level:

import bz2
import gzip
import lzma

so importing any part of pooch failed entirely on such interpreters:

File ".../pooch/processors.py", line 15, in <module>
    import lzma
ModuleNotFoundError: No module named '_lzma'

even for users who never touch Decompress.

Change

As suggested in the issue, guard the three imports so a missing module
becomes None instead of crashing at import time:

try:
    import lzma
except ImportError:
    lzma = None

The module is then only required when the matching Decompress method is
actually used. Decompress._compression_module now raises a clear
ValueError naming the missing module instead of failing later with an
AttributeError on None:

Could not decompress 'data.xz' because the 'lzma' module is not available
in this Python installation. This usually means Python was built without
'lzma' support. ...

When the modules are present (the normal case) behaviour is unchanged.

Tests

• test_decompress_unavailable_module — monkeypatches the module table to
  None (simulating a Python built without the module) and checks the clear
  error for the lzma/xz/bzip2/auto paths.
• test_processors_import_without_optional_modules — spawns a subprocess
  that blocks importing lzma/bz2 and asserts import pooch.processors
  still succeeds (faithful reproduction of lzma import failed in Python 3.13 #468).

Both fail on main and pass with this change; the full
test_processors.py suite (44 tests) passes and ruff is clean.