added pyproject.toml and updated build instructions

CHANGES

@@ -4,6 +4,7 @@ Version 2.7.4 ->
   - added wrappers for UCurrNameStyle and for ucurr_getName() onto CurrencyUnit
   - added support for ICU 70.1
   - added FieldPosition-less overloads to RuleBasedNumberFormat.format()
+  - added pyproject.toml file to (also) build via python -m build
   - 
 
 Version 2.7.3 -> 2.7.4

README.md

@@ -86,7 +86,7 @@ pre-built binary packages of ICU and PyICU, see below.
 Before building PyICU the ICU libraries must be built and installed. Refer
 to each system's [instructions](https://unicode-org.github.io/icu/userguide/icu4c/build.html) for more information.
 
-PyICU is built with setuptools:
+PyICU is built from sources with ``setuptools`` or with ``build`` and ``pip``:
 
    - verify that ``pkg-config`` is available (the ``icu-config`` program is
      [deprecated](http://userguide.icu-project.org/howtouseicu#TOC-C-Makefiles)
@@ -100,12 +100,18 @@ PyICU is built with setuptools:
      Starting with ICU 60, ``-std=c++11`` must appear in your CFLAGS or be the
      default for your C++ compiler.
 
-   - build and install pyicu
+   - **either** build and install PyICU with ``setuptools``
      ```sh
      python setup.py build
      sudo python setup.py install
      ```
 
+   - **or** build PyICU with ``build`` and install it with ``pip``
+     ```sh
+     python -m build
+     sudo pip install dist/PyICU-<version>-<platform>.whl
+     ```
+
 ## Running PyICU
 
   - Mac OS X

pyproject.toml

+
+[build-system]
+requires = [
+    "setuptools",
+]
+build-backend = "setuptools.build_meta"