setup: copy pythonXY.dll next to the hg.exe wrapper when building

This avoids the problem of having the newly built binary complaining that it
can't find the DLL.  There is an option in the python.org installer to add the
python install to PATH (which defaulted to "on" with py2, and therefore was not
an issue up to this point), but that makes switching between python versions
harder.

This shouldn't be an issue with the PyOxidizer binary, but that current has
issues running some of the tests, and took noticeably longer to build last time
I tried it.

Differential Revision: https://phab.mercurial-scm.org/D9362

setup.py

@@ -782,6 +782,9 b' class buildhgexe(build_ext):'
 
         pythonlib = None
 
+        dir = os.path.dirname(self.get_ext_fullpath('dummy'))
+        self.hgtarget = os.path.join(dir, 'hg')
+
         if getattr(sys, 'dllhandle', None):
             # Different Python installs can have different Python library
             # names. e.g. the official CPython distribution uses pythonXY.dll
@@ -807,6 +810,19 b' class buildhgexe(build_ext):'
                     )
                 pythonlib = dllbasename[:-4]
 
+                # Copy the pythonXY.dll next to the binary so that it runs
+                # without tampering with PATH.
+                fsdecode = lambda x: x
+                if sys.version_info[0] >= 3:
+                    fsdecode = os.fsdecode
+                dest = os.path.join(
+                    os.path.dirname(self.hgtarget),
+                    fsdecode(dllbasename),
+                )
+
+                if not os.path.exists(dest):
+                    shutil.copy(buf.value, dest)
+
         if not pythonlib:
             log.warn(
                 'could not determine Python DLL filename; assuming pythonXY'
@@ -829,8 +845,6 b' class buildhgexe(build_ext):'
             output_dir=self.build_temp,
             macros=macros,
         )
-        dir = os.path.dirname(self.get_ext_fullpath('dummy'))
-        self.hgtarget = os.path.join(dir, 'hg')
         self.compiler.link_executable(
             objects, self.hgtarget, libraries=[], output_dir=self.build_temp
         )