Support pathological tmpdirs in TexManager.

tex does not support paths that contain tildes, and tmpdir may be such a
path.  Fortunately, we can arrange to refer to paths relatively instead.

No test, as that would involve setting up another MPLCONFIGDIR just for
that purpose, which is rather costly as we'd need to regen the font
cache for it.  But one can try e.g.
```
MPLCONFIGDIR=/tmp/has~tilde python -c 'from pylab import *; figtext(.5, .5, "a", usetex=True); show()'
```

Author: anntzer

lib/matplotlib/texmanager.py

@@ -258,16 +258,19 @@ def make_dvi(self, tex, fontsize):
         basefile = self.get_basefile(tex, fontsize)
         dvifile = '%s.dvi' % basefile
         if not os.path.exists(dvifile):
-            texfile = self.make_tex(tex, fontsize)
+            texfile = Path(self.make_tex(tex, fontsize))
             # Generate the dvi in a temporary directory to avoid race
             # conditions e.g. if multiple processes try to process the same tex
             # string at the same time.  Having tmpdir be a subdirectory of the
             # final output dir ensures that they are on the same filesystem,
-            # and thus replace() works atomically.
+            # and thus replace() works atomically.  It also allows referring to
+            # the texfile with a relative path (for pathological MPLCONFIGDIRs,
+            # the absolute path may contain characters (e.g. ~) that TeX does
+            # not support.)
             with TemporaryDirectory(dir=Path(dvifile).parent) as tmpdir:
                 self._run_checked_subprocess(
                     ["latex", "-interaction=nonstopmode", "--halt-on-error",
-                     texfile], tex, cwd=tmpdir)
+                     f"../{texfile.name}"], tex, cwd=tmpdir)
                 (Path(tmpdir) / Path(dvifile).name).replace(dvifile)
         return dvifile