casting.py: Filter WSL1 + np.longdouble warning

This commit filters the following warning:

> UserWarning: Signature b'\x00\xd0\xcc\xcc\xcc\xcc\xcc\xcc\xfb\xbf\x00\x00\x00\x00\x00\x00' for
> <class 'numpy.longdouble'> does not match any known type: falling back to type probe function.
> This warnings [sic] indicates broken support for the dtype!
>  machar = _get_machar(dtype)

 To ensure that this warning is only filtered on WSL1, we try to detect WSL
 by checking for a WSL-specific string from the uname, which appears to be
 endorsed by WSL devs.
 (microsoft/WSL#4555 (comment))

 I also tried checking the `WSL_INTEROP` and `WSL_DISTRO_NAME` environment
 variables as suggested in the above linked issues, but I preferred reusing
 the `platform` module that was already imported inside `casting.py`.

 There is perhaps a more thorough approach where we collect all raised warnings,
 test the collected warnings, etc. but I didn't want to overcomplicate things.

nibabel/casting.py

@@ -6,7 +6,7 @@
 from __future__ import annotations
 
 import warnings
-from platform import machine, processor
+from platform import machine, processor, uname
 
 import numpy as np
 
@@ -274,7 +274,15 @@ def type_info(np_type):
             nexp=None,
             width=width,
         )
-    info = np.finfo(dt)
+    # Mitigate warning from WSL1 when checking `np.longdouble` (#1309)
+    # src for '-Microsoft': https://github.com/microsoft/WSL/issues/4555#issuecomment-536862561
+    with warnings.catch_warnings():
+        if uname().release.endswith('-Microsoft'):
+            warnings.filterwarnings(
+                action='ignore', category=UserWarning, message='Signature.*numpy.longdouble'
+            )
+        info = np.finfo(dt)
+
     # Trust the standard IEEE types
     nmant, nexp = info.nmant, info.nexp
     ret = dict(