Fix source installation instructions.

- Replace outdated 'python setup.py install' with 'python -m pip install -e .'.
- Add virtual environment instructions for PEP 668 compatibility.
- Update the GitHub URL to HTTPS.

README.rst

@@ -30,9 +30,23 @@ To install with pywin32 or pyobjc required specify the extra `nativeLib`:
 
     python -m pip install -U send2trash[nativeLib]
 
-or you can download the source from http://github.com/arsenetar/send2trash and install it with::
+or download the source from https://github.com/arsenetar/send2trash and install it with:
 
-    >>> python setup.py install
+    >>> python -m pip install -e .
+
+On systems where Python enforces PEP 668 (e.g., recent Linux distributions),
+installing packages into the system Python may be restricted.
+Use a virtual environment:
+
+    >>> python -m venv .venv
+
+    GNU/Linux / macOS:
+
+        >>> source .venv/bin/activate
+
+    Windows:
+
+        >>> .venv\Scripts\activate
 
 Usage
 -----