INSTALL.md: using Python 3.8 does not work, issue lbryio#2769

Because of issue lbryio#2769 at the moment the `lbrynet` daemon
will only work correctly with Python 3.7.

The `deadsnakes` personal package archive (PPA) provides
Python 3.7 for Ubuntu distributions that no longer have it
in their official repositories like 18.04 and 20.04.

If Python 3.8+ is used, the daemon will start but the RPC server
may not accept messages, returning the following:
```
Could not connect to daemon. Are you sure it's running?
```

INSTALL.md

@@ -9,9 +9,17 @@ Here's a video walkthrough of this setup, which is itself hosted by the LBRY net
 
 ## Prerequisites
 
-Running `lbrynet` from source requires Python 3.7 or higher. Get the installer for your OS [here](https://www.python.org/downloads/release/python-370/).
+Running `lbrynet` from source requires Python 3.7. Get the installer for your OS [here](https://www.python.org/downloads/release/python-370/).
 
-After installing python 3, you'll need to install some additional libraries depending on your operating system.
+After installing Python 3.7, you'll need to install some additional libraries depending on your operating system.
+
+Because of [issue #2769](https://github.com/lbryio/lbry-sdk/issues/2769)
+at the moment the `lbrynet` daemon will only work correctly with Python 3.7.
+If Python 3.8+ is used, the daemon will start but the RPC server
+may not accept messages, returning the following:
+```
+Could not connect to daemon. Are you sure it's running?
+```
 
 ### macOS
 
@@ -31,14 +39,18 @@ Assistance installing Python3: https://docs.python-guide.org/starting/install3/o
 
 ### Linux
 
-On Ubuntu (16.04 minimum, we recommend 18.04), install the following:
+On Ubuntu (we recommend 18.04 or 20.04), install the following:
 
 ```
 sudo add-apt-repository ppa:deadsnakes/ppa
 sudo apt-get update
 sudo apt-get install build-essential python3.7 python3.7-dev git python3.7-venv libssl-dev python-protobuf
 ```
 
+The `deadsnakes` personal package archive (PPA) provides Python 3.7
+for those Ubuntu distributions that no longer have it in their
+official repositories.
+
 On Raspbian, you will also need to install `python-pyparsing`.
 
 If you're running another Linux distro, install the equivalent of the above packages for your system.