Clarify installation instructions

With some of the packaging issues fixed the installation instructions can be simplified.

In particular, `pyproject.toml` and fixed `setup.py` mean that a simple `pip install hdbscan` is now enough, assuming the user has a relatively recent `pip`. Note that the default pip on many versions of Python is pip 8, which is too old. Pip does bug to be updated, but many users do not do that. Hence there is a note that if there is an issue to first upgrade pip then try again.

Another feature of pip is that it can pull directly from GitHub. This can be useful if a new feature is merged but not yet pushed to PyPI. The one-liner pip install is more convenient than manual download and installation.

Author: alugowski

README.rst

@@ -173,7 +173,7 @@ Easiest install, if you have Anaconda (thanks to conda-forge which is awesome!):
 
     conda install -c conda-forge hdbscan
 
-PyPI install, presuming you have sklearn and all its requirements (numpy and scipy) installed:
+PyPI install, presuming you have an up to date pip:
 
 .. code:: bash
 
@@ -182,41 +182,42 @@ PyPI install, presuming you have sklearn and all its requirements (numpy and sci
 Binary wheels for a number of platforms are available thanks to the work of
 Ryan Helinski <[email protected]>.
 
-If pip is having difficulties pulling the dependencies then we'd suggest installing
-the dependencies manually using anaconda followed by pulling hdbscan from pip:
+If pip is having difficulties pulling the dependencies then we'd suggest to first upgrade
+pip to at least version 10 and try again:
 
 .. code:: bash
 
-    conda install cython
-    conda install numpy scipy
-    conda install scikit-learn
+    pip install --upgrade pip
     pip install hdbscan
 
-For a manual install get this package:
+Otherwise install the dependencies manually using anaconda followed by pulling hdbscan from pip:
 
 .. code:: bash
 
-    wget https://github.com/scikit-learn-contrib/hdbscan/archive/master.zip
-    unzip master.zip
-    rm master.zip
-    cd hdbscan-master
-
-Install the requirements
+    conda install cython
+    conda install numpy scipy
+    conda install scikit-learn
+    pip install hdbscan
 
-.. code:: bash
 
-    sudo pip install -r requirements.txt
-    
-or
+For a manual install of the latest code directly from GitHub:
 
 .. code:: bash
 
-    conda install scikit-learn cython 
+    pip install --upgrade git+https://github.com/scikit-learn-contrib/hdbscan.git#egg=hdbscan
 
-Install the package
+
+Alternatively download the package, install requirements, and manually run the installer:
 
 .. code:: bash
 
+    wget https://github.com/scikit-learn-contrib/hdbscan/archive/master.zip
+    unzip master.zip
+    rm master.zip
+    cd hdbscan-master
+    
+    pip install -r requirements.txt
+    
     python setup.py install
 
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