Revert "find sphinx-build before calling build.py"

This reverts commit 07200f44 because
the latter doesn't work with virtualenv correctly.

doc/CMakeLists.txt

@@ -4,13 +4,8 @@ if (NOT DOXYGEN)
   return ()
 endif ()
 
-find_package(PythonInterp
-  QUIET REQUIRED)
-find_program(SPHINX_EXECUTABLE
-  NAMES sphinx-build sphinx-build-3)
 add_custom_target(doc
-  COMMAND ${CMAKE_COMMAND} -E env SPHINX_EXECUTABLE=${SPHINX_EXECUTABLE}
-    ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/build.py ${FMT_VERSION}
+  COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/build.py ${FMT_VERSION}
   SOURCES api.rst syntax.rst build.py conf.py _templates/layout.html)
 
 install(DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/html/

doc/build.py

@@ -100,8 +100,7 @@ def build_docs(version='dev', **kwargs):
     raise CalledProcessError(p.returncode, cmd)
   html_dir = os.path.join(work_dir, 'html')
   main_versions = reversed(versions[-3:])
-  sphinx_build = os.environ.get('SPHINX_EXECUTABLE', 'sphinx-build')
-  check_call([sphinx_build,
+  check_call(['sphinx-build',
               '-Dbreathe_projects.format=' + os.path.abspath(doxyxml_dir),
               '-Dversion=' + version, '-Drelease=' + version,
               '-Aversion=' + version, '-Aversions=' + ','.join(main_versions),