Adding documentation to the README for import errors in vim (#1317)

* Adding documentation to the README for import errors in vim

I had the same issue as psf/black#1148 and have been searching for a
solution to this. I realized that you cannot fix it by change anything
in the code, but by re-compiling the C extensions of regex and
typed-ast. Installing this packages from the tarballs solves the
problem.

* Fixing a bad copy&paste

Co-Authored-By: Hugo van Kemenade <hugovk@users.noreply.github.com>

* chore: changed made by pre-commit

* chore: better way of dealing with non-binary installs

* chore: adding vim cache files to the ignore list

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

.gitignore

@@ -13,3 +13,4 @@ _black_version.py
 .idea
 .eggs
 .dmypy.json
+*.swp

README.md

@@ -823,6 +823,55 @@ default. On macOS with Homebrew run: `brew install vim`. When building Vim from
 use: `./configure --enable-python3interp=yes`. There's many guides online how to do
 this.
 
+**I get an import error when using _Black_ from a virtual environment**: If you get an
+error message like this:
+
+```text
+Traceback (most recent call last):
+  File "<string>", line 63, in <module>
+  File "/home/gui/.vim/black/lib/python3.7/site-packages/black.py", line 45, in <module>
+    from typed_ast import ast3, ast27
+  File "/home/gui/.vim/black/lib/python3.7/site-packages/typed_ast/ast3.py", line 40, in <module>
+    from typed_ast import _ast3
+ImportError: /home/gui/.vim/black/lib/python3.7/site-packages/typed_ast/_ast3.cpython-37m-x86_64-linux-gnu.so: undefined symbool: PyExc_KeyboardInterrupt
+```
+
+Then you need to install `typed_ast` and `regex` directly from the source code. The
+error happens because `pip` will download [Python wheels](https://pythonwheels.com/) if
+they are available. Python wheels are a new standard of distributing Python packages and
+packages that have Cython and extensions written in C are already compiled, so the
+installation is much more faster. The problem here is that somehow the Python
+environment inside Vim does not match with those already compiled C extensions and these
+kind of errors are the result. Luckily there is an easy fix: installing the packages
+from the source code.
+
+The two packages that cause the problem are:
+
+- [regex](https://pypi.org/project/regex/)
+- [typed-ast](https://pypi.org/project/typed-ast/)
+
+Now remove those two packages:
+
+```console
+$ pip uninstall regex typed-ast -y
+```
+
+And now you can install them with:
+
+```console
+$ pip install --no-binary :all: regex typed-ast
+```
+
+The C extensions will be compiled and now Vim's Python environment will match. Note that
+you need to have the GCC compiler and the Python development files installed (on
+Ubuntu/Debian do `sudo apt-get install build-essential python3-dev`).
+
+If you later want to update _Black_, you should do it like this:
+
+```console
+$ pip install -U black --no-binary regex,typed-ast
+```
+
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